A levels don't measure intelligence, they measure education. If cars are better made than they were 50 years ago (and they certainly are), I don't think it's impossible that children are better taught now than they were then. It's difficult to compare, though, because they're taught different things (we didn't have computer training in my day, so more time to do history) and with different goals (success in education seems now to be calculated by how many graduates get jobs).
This writer sat an A-level at 37 and reports back that there were good things and bad things (her subject was English and of course no one subject can be representative of experiences in others):
http://www.guardian.c...17/took-a-level-at-37