Dating a 25 year old
You can not tell her to stop seeing her boyfriend, and you should not warn her from her boyfriend only to be able to tell her "I told you so" when/if things go south.
What you can do, depending on your relationship to her, is to share your concerns, while acknowledging that they are irrational.
It's not likely that anything worse will happen to her with this "sensible" 25 year old, than what would happen with a random 17 year old.
Your daughter is an adult now, in all but the legal sense, so treat her like an adult.
It's like a drug and despite parents, friends, or even common sense whispering behind the scenes 'don't do this'.
She continues because the feelings she has are too strong. But I would be careful in how you deliver your feelings. Outline your concerns but let your daughter know you love and support her and that it is only natural for you to be worried.
She went to high school and lived with her parents.
Since then almost 4 years past and we live together in another city and we are both happy and in love.
You sound like a great parent and I wish the very best to you and your daughter. While such concerns are natural, and perfectly understandable, they are also irrational.
The general rule of thumb for age appropriateness is half plus 7.