There are lots of nice apartment complexes and plenty of bars and restaurants and I wouldn't say there was any type of 18-30 scene. There may be one or two clubs and bars which stay open until the small hours but they're easy enough to avoid if you're not into that. We were in Caleta at Easter (our second time there) and had a great time going out every night and meeting lots of fantastic people in nice family-friendly bars with good music/entertainment. From what I can gather, Corralejo is very similar.
As for being the quietest island - people who love Tenerife/Benidorm/Faliraki etc usually complain about there being nothing to do in Fuerteventura - there is actually plenty to see and do but most of it doesn't involve clubbing
Weather - it will be nice whenever you go, although in recent years the winter months have been a little more cloudy and damp, but this doesn't mean you won't come with a fantastic tan!

Apart from La Palma its the quietest Island but has the best beaches If you are wanting a quiet resort I would suggest Costa Calma it is however predominantly German.
We have tended to go in November and weather has usually been mid 70s
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thanks for the replies anita