Recognize the powerful grip the 25 yr old has and that is normal. I would try and create a containment bubble around a situation you have limited control over but in reality you do have a way to contain the situation. Reinforce her education about the risks of getting pregnant and maybe set some soft rules like 'education comes first' Maybe she is in sixth form. It could be that the relationship is successful but if something goes wrong be sure she knows you are there for her if things collapse.
I get myself sick with worry in regards to my kids.
The obligatory XKCD cartoon: 25 and 17 is slightly over. He owns his own successful business although he still lives with parents.I know its only 8 years difference but it's the difference between him being at an age where he must be thinking about starting a family, marriage, etc and her starting out in life. Do I let them get on with it or should I try to explain my above concerns at the risk of pushing them together?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.However, generally speaking women mature earlier than men. they aren't (yet) setting up a home and living together.