It’s totally straightforward.

You send a sample of your work, along with an application. Our jury of professional craftspeople will assess your degree of skill at its present level, and then do one of two things:

• assign you a ranking, or
• suggest that you work a bit longer and
  re-submit your work again later

You can reapply for guild membership up to three times in a calendar year for the same application fee.

By the way, your age has absolutely no bearing on your qualifications or eligibility (we never ask), since your workmanship speaks for itself.