Although age is something that is part of it for me, I tend to more think of a twink being defined by their body type. Here is something that I came across that seems, for the most part, pretty good at defining the various terms (ref: https://pairedlife.com/dating/Gay-Men-Are-you-a-Jock-Otter-Bear-or-Wolf):
Gay Body Types
[TABLE]
[TR]
[TH]Type[/TH]
[TH]Build[/TH]
[TH]Hair[/TH]
[TH]Age[/TH]
[TH]Example Celebrity[/TH]
[TH]Notes[/TH]
[/TR]
[TR]
[TD]Otter[/TD]
[TD]Thin or athletic[/TD]
[TD]Lots[/TD]
[TD]Any age[/TD]
[TD]Scott Caan[/TD]
[TD]Part of the extended bear community[/TD]
[/TR]
[TR]
[TD]Wolf[/TD]
[TD]Lean, muscular[/TD]
[TD]Semi-Hairy[/TD]
[TD]Any age[/TD]
[TD]Joe Manganiello[/TD]
[TD]Sexually aggressive; Silver or grey wolf is an older wolf[/TD]
[/TR]
[TR]
[TD]Bear[/TD]
[TD]Big, often with a belly[/TD]
[TD]Lots[/TD]
[TD]Any[/TD]
[TD]John Travolta[/TD]
[TD]Very masculine[/TD]
[/TR]
[TR]
[TD]Cub[/TD]
[TD]Husky[/TD]
[TD]Lots[/TD]
[TD]Young or younger-looking[/TD]
[TD]Jack Black[/TD]
[TD]Sugar cubs and muscle cubs are sub-types[/TD]
[/TR]
[TR]
[TD]Chub[/TD]
[TD]Real big[/TD]
[TD]Maybe[/TD]
[TD]Any[/TD]
[TD]John Goodman[/TD]
[TD]Distinct from bears[/TD]
[/TR]
[TR]
[TD]Pup[/TD]
[TD]Slender[/TD]
[TD]No[/TD]
[TD]Young[/TD]
[TD]Joe Jonas[/TD]
[TD]Energetic, cute, and naive[/TD]
[/TR]
[TR]
[TD]Bull[/TD]
[TD]Super-built[/TD]
[TD]Maybe[/TD]
[TD]Any[/TD]
[TD]Dwayne "The Rock" Johnson[/TD]
[TD] [/TD]
[/TR]
[TR]
[TD]Twink[/TD]
[TD]Slender[/TD]
[TD]No[/TD]
[TD]Young[/TD]
[TD]Justin Beiber[/TD]
[TD]Self-centered, usually between 18 and the mid-20’s[/TD]
[/TR]
[TR]
[TD]Twunk[/TD]
[TD]Muscular but slender[/TD]
[TD]No[/TD]
[TD]Young[/TD]
[TD]The muscular version of Beiber[/TD]
[TD]A more muscular Twink[/TD]
[/TR]
[TR]
[TD]Gym Bunny[/TD]
[TD]Sculpted[/TD]
[TD]Maybe[/TD]
[TD]Under 50[/TD]
[TD]Shemar Moore[/TD]
[TD]Fitness associated with gym, not sports[/TD]
[/TR]
[TR]
[TD]Jock[/TD]
[TD]Muscular and athletic[/TD]
[TD]Maybe[/TD]
[TD]Any[/TD]
[TD]David Beckham[/TD]
[TD]Fitness is associated with sports[/TD]
[/TR]
[TR]
[TD]Gym Rat[/TD]
[TD]Very lean and well-built[/TD]
[TD]Maybe[/TD]
[TD]Any[/TD]
[TD] [/TD]
[TD]Addicted to working out[/TD]
[/TR]
[/TABLE]