Now that you've written it out I can tell that the "-ty" of "fifty" was probably shortened from some older pronunciation more resembling the word "ten". But I guarantee you that native English speakers are not thinking this in large numbers. This is the sort of insight that comes better when it's not your native language, others of us will just internalize "fifty" as its own word.
I guess it's also no coincidence that words ending in -teen are frequently confused with and misheard for those ending in -ty.
Edit: Seems like they derive from different Germanic roots for 10.
It's no where near as clear as the chinese version which is literally five tens though. To know fifty is 50 and five tens you have to already know both the meaning of number places and what fifty means, really hard to derive from just the name or seeing 50 written out.
It's not super hidden but you also already understood numbers at that age too presumably so you already had most of the puzzle. As a native speaker I don't think I ever broke the words down like that or had them broken down to me like that (then again this would have been ages ago so maybe it was and I don't remember).
As a native speaker I'm with you. Always seemed pretty obvious to me too, especially above 5 with six-ty seven-ty etc. To be fair though, Chinese says/writes 15 as ten-five which is much more obvious than the difference between five-ty and five-teen. It's interesting in spoken English the emphasis flips between first to second syllable though.
