Incuriously, all you could find was "this is just cultural/ how the language evolved" because that's all there is to it: this is about the nature of language, not logic.
The French say "I have Y years” because theirs is a Romance language, based on Latin. The English use “I am Y years old” because like modern German, theirs is anciently a Germanic tongue…
Volumes have been written on the differences and they are for degree-level study, not SE forums.
The very words “language” and “tongue” are, respectively, Romance and Germanic and while many of us know they are almost perfectly synonymous, at the same time all of us know they are very, very different.
Since both seem to matter what’s your own native tongue, please? Will you say which “most” languages you know of, or Edit the Question to leave that out?
No logic is needed to make the construction "I am X years old" possible. If it was, no such idea could ever come from “the fact that, in English, we modify/specify words by placing words before the word we want to modify/specify” for the simple reason that that’s not true; we might sometimes, but never necessarily.
Ignoring any idea of “logic” will you please consider the very different example of a person walking towards a door?
English speakers say simply “I approach the door.”
French speakers say “I approach myself to the door.” (“je m'approche (de) la porte…”
Exact translation might never be possible, precisely because the languages do not work the same way; do not follow the same “logic”
Native English speakers never "say…" their age. They either "state…" their age, or "say that their age is…" Does that difference matter, or help?
More purely Germanic languages do, and to an extent ancient English did insist but modern English has no real interest in word order. There is no reason for modern English to choose "That was done well" over "That was well done" whatever anywhen German, ancient Latin or modern French suggest.
What track you’re on isn’t clear but in your example "900 years old” might well “behave”, though it could too rarely “be behaving” in the same way as "very old” in English, and then not generally but only in particular circumstances.
What information you’ve found about numerals as adjectives or adverbs might be as conflicting as possible and to that extent, it necessarily is nothing like “the available information…” Sorry.
Nothing more specific about numbers, or this particular construction, applies here and still, all you could find was "this is just cultural/ how the language evolved" because that’s all there is to it: this is about the nature of language, not logic.