The general rule of thumb for age appropriateness is half plus 7.
The obligatory XKCD cartoon: 25 and 17 is slightly over.
That way you don't force her to change her life, yet still make her aware of the concerns.
Several answers already, but I want to address a couple of your concerns.
I have just found out that my 17 year old daughter is going out with a local 25 year old. They haven't shared the same live experiences, they'll soon want different things, etc. I'm concerned that she'll get hurt, pregnant or that, even if they are truly in love, she'll end up growing up too quickly and miss out on things girls her age do like university, traveling, and building a career.