The only trouble is the older I get and harder I work, that by the time the day arrives, I'll probably want to stay in!
You will indeed, if our experience is anything to go by!
Agreed that it should be according to maturity rather than age. You also have to take the dynamics of the various combinations into account. Our 13 (going on 40) year old is mature and sensible enough to look after either one of her younger siblings, but I would be reluctant to inflict both of them on her, due to their ability to fall out (with each other) over almost anything! However, their big sis of 17 (known at home as Nanny McPhee) was quite capable of managing all three from about 15 - mainly due to relative maturity of second child, who - as second in command and in training to take over as "eldest" when big sis goes to uni - often proves a useful ally when it comes to enforcing bed time!
Having said that though - for reasons outlined by the OP - now that we are able to go out, we hardly ever do.