Dniece (18) has boyfriend who's 27

[avenueq]

Would that concern you? Apart from the fact that of course it's not really my business

Not really, no. It depends wholly on the individuals involved, imo, and the nature of their relationship.

Well it's legal but I would find it a bit odd.

At 27 how much do you have in common with a teenager? She would have to be mature and him very innocent for them to meet in the middle, otherwise I would think him a bit odd.

I am visualising DS at 24 dating my friends 18 year old and they are light years apart in interests, education, work level, finances etc

I wouldn't worry about that at all. Many people I know have similar age gaps. They are older now but met when e.g. 20 + 30

I met DH when I was 18 and he was 27. That was 42 years ago.
However I would still be cautious because in our case I think we met in the middle maturity wise.