At what age is a guinea pig fully grown?

A guinea pig is generally fully grown at about 5 to 6 months of age, meaning its bones and internal organs have finished developing. However, at this stage the guinea pig is usually on the slim side, and its body shape firms up a bit later.

An adult guinea pig typically weighs about 700 to 1200 g and measures around 20 to 25 cm in length. After maturity, the owner should make timothy hay the staple diet; continuing to feed large amounts of alfalfa hay can easily lead to obesity.