In my own experience, a responsible 14/15-year-old whom your kids know and like can often be better than an 18/25-year-old who's just doing it for the money. Even for an overnight — on condition of course that the 'Cavalry' are on hand if anything goes seriously wrong...
I'm not sure that we're answering the OP's question, 'though — if you're asking whether there's an official, 'legal age at which children can be left at home alone', the short answer is probably none. Until they're 18, the parent/guardian is responsible for their well-being, afaik, and of course the extent to which their safety/well-being might be compromised by being left at home alone depends on the circumstances.
A cautionary tale — I recently heard of a woman living near us who had a dreadful experience in this regard. She had a really important appointment to keep, and her babysitter was delayed but had called to say she'd definitely be there within the half-hour — so she left her 11-year-old and 8-year-old in front of the telly, with strict instructions about not answering the door, etc. and went ahead. 15 minutes later the Social Services arrived — having been called by a 'concerned neighbour' — and removed the children from the home. She wasn't allowed even see her children for a further six hours, and they weren't released back into her care until the following day...
I/we had to wonder if it is 'cos she is black'...? :mad: