This is quite fine.
Actually, when I read your first question, what came to my mind first was １８歳の.
Using で sounds a bit awkward to me.
If it is followed by a phrase using verb, I would not feel this awkwardness, perhaps.
Using の makes two adjective work towards the noun with the same strength (?).
18歳の男の子
アメリカの男の子
↓
18歳のアメリカ人の男の子
(I'd use アメリカ人の rather than アメリカの）
で is more like "and then" and separating "being 18 years old" from "American boy".
I might use で in this way:
e.g.
私は18歳のアメリカ人の男の子で、今日本語を勉強しています。