When does a beard start becoming thick and full?

Depends: Different men's skin responds differently to androgens. Some men never develop a thick beard or chest hair, but have normal adult male sexual function; these men usually don't have baldness later. Usually a man can tell if he'll have a thick beard by age 17, almost certainly by age 21. Some women prefer a thick-bearded hirsute man, some a smooth man, most really care mostly who you are.

Varies: It varies. Some people never get a full beard to grow.