Yea not sure I understand the question.If you haven't kept any kind of track since it broke soil,one way is this.When a plant is immature,its nodes or branches like up evenly along the stalk.If you have a plant where the branches are staggered and not lining up evenly along the main stalk,it is older or more mature.These are signs of sexual maturity actually.So if you have two plants and both are 24 inches tall but one has branches that line up evenly parallel along the stalk it would be younger than a 24 inch plant where the uppermost nodes or branches are staggered,another words not lining up at the exact same height along the main stalk.Other than that you have to keep track!
What can you think of to determine a plants age besides the obvious like size or stalk thickness?I grow the huge majority from clones but I have definatly observed this pattern in my seedplant grows.
Ya that...yeehaw....when it shows sex it's mature otherwise...grow till you are ready to flowerEdit: I miss read the question. If I were to tell someone the age of a plant, I would count from when they are standing up with fully open cotyledon. Everything before that I would consider part of germination, everything after is growth.
to see if im behind (stunted) or not.