There is a positive way to look at it: overall prison population doesn't increase in the UK, and if the elderly make higher and high fraction of inmates, it suggests that younger people aren't as criminally inclined as the old generation.
Or the police became less efficient over time (extreme model: all 20 year old criminals used to get caught, but now only 5% of them do) or prison sentences got longer over time (most criminals get less criminal as they get older. A simple model has only people under 50 getting prison sentences. If those all used to get sentenced to 10 years, but nowadays get sentenced to 20 years, the number of over 60 year old prisoners went up from zero)
My pet theory is all the young guys who used to get busted for things they did because they were bored are now sitting at home playing GTA.
more evidence for the lead poisoning theory?
Quite likely. While numerous explanations exist, none of which sounds intuitive.