She says her friends won't understand the age gap: what to say

She brought up her friends, which means this is probably about her own social world more than about you. The most useful thing to figure out is which friend, which comment, or which imagined reaction is doing the work. A vague sense that people will think it's weird is harder to act on than a specific memory of someone rolling their eyes at a coworker's partner.

Don't assume she's ending things by bringing it up. Plenty of people raise this early because they are thinking ahead and want to see how you handle it. Her tone and how much she engages in the follow-up will tell you more than the first line does.

The question that moves this forward is some version of what do you think they'd actually say. It sounds simple, but it makes her get concrete, and concrete worries are easier to talk through than general dread. She might admit it's mostly one friend, or that she's worried about her mum, or that she has no idea and is anxious about not knowing.

Once she names something, you can respond to that thing. If it is a specific friend who is protective, you can talk about what would make that friend more comfortable over time. If it is a general fear, you can talk about the two of you and how you'd handle questions, without promising that nobody will ever have an opinion.

Her friends are not your audience, and you shouldn't write your replies as if they were. But she is trying to figure out whether this is worth the friction, and that is a real decision she gets to make. Make it easy for her to take her time. Say that you understand she might need to think about how her people will react, and that you're fine either way.

That last part matters. If you are visibly anxious about losing her, she has to manage your feelings on top of her own. Keep your tone steady, let the pause exist, and avoid asking follow-up questions every hour about where she landed.