Just got back from a Donnington Park Cycle Night - went with my 10 year old son, a mate and his daughter.
Absolute hoot, several hundred riders - and one rollerblader - doing laps of the Donnington circuit. Every age from tots on stabilisers through to a group of more mature gentlemen in tweeds riding Raleigh Shoppers. Lots of MAMILS, lots of MTBers, riding everything from 0-10" travel at either end.
And everyone having a great time just riding whatever bikes they had in whatever manner they liked. The best £1 entry fee I have ever spent!