You have already answered your own question there Alex.
There used to be a minimum age of 25 for Classic Car Insurance many years ago but I did get a policy at the ripe old age of 23. I would imagine you could get cover at 21 with a clean licence and some NCD but why not send emails to the car clubs who may be able to help you. They will probably have details of schemes that high street brokers don't have on their books.
Good luck with your Classic Car....