Of course, nobody can 100% prove anything at all. However, it's possible to tell if a user is underaged beyond a reasonable doubt simply by watching their behaviour.
If a user more than a few of the things on the short checklist that follows, then he is most likely an underaged user beyond a reasonable doubt.
- Extremely immature (should be relatively self-explanatory)
- Underdeveloped sense of humor (thinking that jokes about shit and the like are funny, calling out Dark Humor as being terrible)
- Saying he's underaged (as you have said, not 100% damning, but is a factor. It should also be obvious that a joke or demonstration is so, as it is with you.)
- Little to no profiecency in English typing and writing conventions (can't write in a manner to be comprehensible to the reader)
- Little to no profiecency in Reading Comprehension (doesn't fuсking listen to your comments because he can't understand them)
Essentially, we CAN prove that a user is underaged.
Caption this Meme 