I first left my eldest when he was three months when OH and I went to a wedding. He was with my parents. I didn't have him stay out overnight until he was about a year. We've not had a huge need to leave him overnight to be honest cos I'm not a massive drinker and my parents are always happy to come to ours to watch the boys if we go out for an evening.
I hated leaving my eldest to begin with but now I'm used to it, I don't mind. With my youngest I'm already thinking about leaving him so OH and I could get out on a date and he's five weeks old. It's definitely easier the earlier you leave them, at this age they don't mind who has them as long as they're fed, changed and cuddled. The older they get the more aware they become of being left and it can be difficult if it's not something they're used to. My eldest loves having his granny and grampa look after him now.