Koh Lanta, although I found it a bit limited on snorkelling; but otherwise ticks all boxes. Snorkelling was wonderful on Koh Lipe but that place is changing very fast and I was there over two years ago, hopefully someone can give a more up to date report.

Hello
My friend and I are mid thirties and are going to be travelling through Thailand for a few weeks. Naturally we are wanting some beach time (maybe 5 or 6 days)...
We were going to go to Ko Phi Phi but have heard reports that it's a bit 18-30s-ish and full of 'brits abroad'... !
Can anyone recommend lovely islands / areas which are more relaxing (yet still beautiful)... (yet not 'resorty' like Phuket) .. some restaurants and a couple bars would be great but we just don't want anywhere too brash.
Access to good snorkelling would be great too.
We were thinking somewhere near Krabi rather than Ko Samui / Ko Pha Ngan ...as I've been to Ko pha ngan before.
Many thanks in advance :)
If you want to stay around the Phuket / Phi Phi area but without the "Brits abroad" scene, have a look at Koh Yao (Noi or Yai). Plenty of unspoilt beaches there. Thorn Tree user Tezza has a very useful blog here.
Phi Phi isnt Brits abroad ! its Skanks Abroad. Dutch, Russians, Aussies, Eurozoners and Brits Scandis and Canadians...basically every nationality....all running round with buckets of booze in 1 hand and condoms in the other - very spring break. Avoid like the plague
Kata and Karon in Phuket are much "older" but try down in Lanta on Khlong Kong or Nin which ar really cool places, nice ambience but quiet at night and the beaches are amaing
When are you planning on going? Depending on when you're going, Koh Lipe might be a good option.
Couldn't help but laugh at baklavalover! Skanks abroad! Brilliant.

I wouldn't say that Koh Phi Phi is brit central, it's backpacker central. I would describe it as KSR on an island. I'm not saying that's good or bad, it's just what it is.
As for those Brits. I was travelling around with 3 Brits and everytime we came upon people fighting or acting poorly at a bar, one of them would say "Please don't let them be British. Please don't let them be British." Unfortunately, 9 times out of 10, they were.
Your question is very strange, I personally think you can find quiet, nice spots on most Thai islands / beaches away from the masses that are not age restrictive.. Few islands are so tiny that you have to share beach-time with absolutely everyone else. "Some restaurants and a couple of bars" are easy to find almost everywhere. Just pick somewhere that fits in with your travel plans / route, they are all much of a muchness and suggestions are subjective and may not suit you at all. Good snorkelling (directly from the beach) in Thailand is harder to find, it's not world class, but Koh Lipe and Koh Phangan / Tao are probably the best I have encountered, but Thailand is not really a snorkelling destination.

Koh Chang and Krabi are 2 of my faves. Can do very good snorkelling trips from either - you need to do a tour but it's cheap. More of an adult crowd.
Phi Phi is drunk loser kid central like fmp rubbish.
Pha Ngan has no decent off shore snorkelling so don't know what unfriendly girl is on about. Tao has that.