his channel description has for a while said he was born in 1998, so tbh i don't really doubt he's 15.
although it isn't great proof, i can show you that right at this very moment his About page still says he's 14 so it must have said 1998 there before he posted this birthday video

i know a dude at my school who is a month older than me (we're both 15), shares the same level of a math class as me, and yet he acts and looks like he's 12 or 13.
sammy here is more mature looking and has a deeper-pitched voice than this friend of mine does, so it's easy for me to believe that sammy is actually 15 but just a late-bloomer for whatever reason.