I personally know of no way to determine exactly how old a guinea pig - ie by checking the teeth like you would a horse etc ! The only guidelines I can give are as follows:
Source: Anatomy of the Guinea Pig - Cooper & Schiller states:
"The guinea pig increases in weight and length until about 15 months - after which, weight changes are secondary to loss or gain in stored body fat.
The weights of mature males range from 800-1600gm and of mature females from 700-1300mm.
The length, measured from nose tip to last vertebra, for mature males is 30-35cm and for mature females 28-32cm." However, in showing terms an 'adult' is classed as 8 months and over.