I've been doing some housecleaning for the sake of correct grammatical conjugation, which includes classifying verbs. Jisho.org's top result doesn't have あげる as meaning 'to give' (granted it's colloquial), it's 上げる, to raise. I don't know if あげる should be classified as an Ichidan (ru) verb or an いる/える Godan (u) verb, mainly because my list of exceptions is slightly incomplete. I am thinking あげる is an Ichidan verb (could 上げる just be the unusual kanji version?), but because of the lack of documentation I've been able to find regarding it I'm honestly not sure.
あげる is an ichidan verb (あげる、あげます、あげた...). On Jisho, "to give" is definition 18; when the verb is used to mean "to give," it's almost never written with kanji.
I'm not sure where you got the idea that あげる is colloquial. It's common and appropriate in speech and writing in nearly all contexts. (A couple of exceptions: when you need to reach for a humble verb, you'll want to say 差し上げる; when giving to an animal or, informally, to a friend, think やる.)