Facts about the animal kingdom

How Old Is My Guinea Pig?

Cristina Pascual
By Cristina Pascual, Veterinaria. Updated: January 29, 2024
How Old Is My Guinea Pig?

Guinea pigs are very active rodents that are fun, non-aggressive and ideal as pets for children. If you have a guinea pig or are thinking of adopting one, you may be wondering how old they are. When guinea pigs are young, it's easy to estimate their age based on height and weight. However, once they reach their adult weight and stop growing, determining our pet's age can become a complicated task. Knowing the actual age of your guinea pigs, however, is crucial because it helps you make informed decisions about their health.

The following AnimalWised article will help you determine your guinea pig's age at different stages of life.

You may also be interested in: Guinea Pig’s Vision - How Guinea Pigs See
  1. How to know the age of a baby guinea pig?
  2. How to know the age of an adult guinea pig?
  3. How long does it take for a guinea pig to grow?

How to know the age of a baby guinea pig?

First, you should know that the gestation period of guinea pigs is relatively long, ranging from 58 to 75 days. The fact that the gestation period is longer than in other rodents (such as hamsters, mice, or rats) means that the young are born highly developed, with hair, open eyes and ears and erupted teeth. In short, they have the same characteristics as an adult specimen, only in a smaller size. Therefore, the most important parameters that we can measure to determine the age of our guinea pig are height and weight.

To measure the size of your guinea pig, you need to place it on a flat surface and place a measuring tape parallel to the body next to it. You should measure from the tip of the nose to the back. If your guinea pig moves around too much and this makes it difficult for you to take an accurate measurement, do not worry, an approximate idea is sufficient. To weigh it, you can use a kitchen scale and place a container on it to put your guinea pig in, so it does not move too much when you weigh it. Make sure the scale is tared before placing your guinea pig on it, otherwise you will not get an accurate result.

Here is a guide on how to calculate the age of a guinea pig based on its size and weight:


Unlike weight, the size of male and female guinea pigs is similar throughout their lives, so you can use these measurements as a reference regardless of gender.

  • At birth, they usually measure about 8-10 cm (3-4 in).

  • At weaning, which typically occurs at two or three weeks of age, they reach 13-15 cm (5-6 in).

  • At 8 weeks, they measure between 15-18 cm (6-8 in).

  • At 18 weeks, they measure between 18-23 cm (7-9 in).

  • At 14 months, the age at which growth is approximately complete, they reach 23-27 cm (9-11 in).


Up to the age of 15 months, the weight of females and males is similar. From then on, males weigh significantly more than females (about 20-25% more).

  • Birth weight is usually between 70 and 120 grams.

  • At weaning, at two or three weeks of age, they reach 150-200 grams.

  • Between 4 and 8 weeks of life, they weigh between 200 and 400 grams.

  • At 18 weeks, they weigh between 600and 900 grams.

  • At 15 months of age, differences between females and males can be seen. At this age, females weigh between 700-900 grams and males between 900-1200 grams. They should maintain this weight for the rest of their lives, keeping in mind that female guinea pigs can double their weight during pregnancy.

In order for guinea pigs to grow properly and stay healthy, they must receive vitamin C, as they cannot synthesize it in their bodies. Therefore, it is important to provide these animals with a diet rich in vitamin C. Continue reading this other article if you want to know more the importance of Vitamin C in guinea pigs.

How to know the age of an adult guinea pig?

Once guinea pigs stop growing and reach their adult weight at about 14-15 months of age, height, and weight are no longer useful parameters to estimate their age. Therefore, determining the age of an adult specimen can be an almost impossible task. However, you can estimate the approximate age of your adult guinea pig based on the following signs:

  • Eyes: Looking at your guinea pig's eyes can tell you how old it is. Young guinea pigs that are less than a year old have large, black, shiny eyes. However, as they age, their eyes become smaller (relative to their body size) and less shiny. In some cases, the black color may even fade and change to a duller shade. The eyes of your guinea pig may also become "sunken" as it ages.

  • Teeth: Guinea pig teeth begin to grow days after birth. Similar to infants, their teeth remain small and thin at first. They also remain white for the first year. But over time, the teeth become larger and yellow. Sometimes the teeth can also develop grooves and even erupt.

  • Nails: the nails of a guinea pig go through different stages with age. Thus, the nails of a young guinea pig are pointed and translucent (or partially translucent) and white in color. But once they are a year old, their nails become thicker and rounder. The translucent color then changes to an opaque yellow.

  • Behavior: Behavior not only tells you your guinea pig's personality. You can also tell how old your pig is by its behavior. Young guinea pigs, for example, tend to be more active and playful. They are also more curious and willing to explore everything. Older guinea pigs tend to be more reserved and less active. They are also less curious and eager to explore. They have seen it all and are not as excited about new things.

Be suspicious if your pig's behavior suddenly changes. For example, be wary if you notice that normally active guinea pigs are suddenly holding back. In these cases, it does not mean they are getting old. It could be a sign of illness. If you want to know more, read this other article, where we tell you how to tell if your guinea pig is sick.

The signs of aging can give us a clue whether our guinea pig is more or less old. However, we must remember that the speed of aging can vary from individual to individual, especially depending on the care they have received during their life.

How long does it take for a guinea pig to grow?

Guinea pigs are very fast-growing animals. As we have seen, they are born with a length of about 8-10 cm (3-4 in) and in only 8 weeks they double the size they were born with. During these first weeks of life, they gain up to 50 grams per week. From this point on, growth slows, although they continue to gain weight and size until they are 14-15 months old. From then on, their growth stops, and they are considered adults.

Once they have finished growing, you will need to ensure that their weight remains stable, as they are animals that are very prone to obesity. Therefore, it is important to monitor their weight weekly and provide them with a balanced diet that includes feeds in appropriate proportions. Remember that excess feed can not only lead to obesity problems, but can also cause dysbiosis and dental disease.

If you want to learn more about the different stages of a guinea pig's life, check out this other article where we discuss guinea pig life expectancy.

If you want to read similar articles to How Old Is My Guinea Pig?, we recommend you visit our Facts about the animal kingdom category.

  • Clemons, DJ (2011). Seeman, JL The laboratory guinea pig. CRC Press.
  • Quesenberry, KE, Donnelly, TM, Mans, C. (2012) Ferrets, rabbits and rodents. Elsevier
  • Reid, M.D. (1958). The guinea pig in research; biology, nutrition, physiology. National Institutes of Health.
Write a comment
Add an image
Click to attach a photo related to your comment
What did you think of this article?
How Old Is My Guinea Pig?