I'm nearly 40 years old and can honestly say one of the happiest times of my life was when living separately but together with DP.
Since moving in, I can say I have had times which have ranked pretty high up on the unhappiest of times in my life. 
Positive things;
More money.
Nice not having to pack bags to go between houses.
It makes other people see you as a "couple" rather than just boyfriend and girlfriend.
Being able to have a cuddle everynight.
Less planning.
Having someone to share mundane chores.
Negative things;
There is only one negative as far as I'm concerned but it's a biggie!
Having to share the crap that step parenting and dealing with a difficult ex and Disney Dad brings. The effect it can have on your life when you realise you have very little control and influence over things that affect you directly on a daily basis is terrible for destroying your self esteem.
I've decorated this house, I've got all my things here, my DS has his own room, but I doubt it will ever feel like my home until I feel I have a say in what goes on here, a REAL say, not just what to cook for dinner or what colour emulsion to slap on the walls.
DP would have finished the relationship eventually if I hadn't moved in, I think he is bothered about what other people think a lot more than me. If I had a straight choice, I would say I should have stayed put.