18 months between season 7 and 8, 6 months between season 8 and 9, I think that she should be closer to 30 than to 25.
Buffy was 22 when S7 ended, so, given the timeline of each season and the space in between, 26 sounds about right. Even a bit too old.
@Kean
It's not that, I don't expect Buffy to die from natural causes either, it's just, people change as they get older. Priorities change. Sure, I don't know if slayers work like this, it's pure speculation but what if Buffy decides one day that she wants to be a mom, wants to start a family? What if age starts affecting her performance as the slayer? What if she's ready to retire but Angel is not? Buffy doesn't have an eternity, that's what I'm trying to say, I guess.
No, I totally get what you're saying but I tend to view these things as conflict to be resolved within the relationship, not reasons for them not to get together at all. So often writers say you can't get a couple together because then you have nowhere to go but relationships are fecking hard and I think there is plenty of story in one involving Buffy and Angel.