ATCO's don't grow on trees - there is a 20% shortfall of qualified radar controllers throughout Europe. If you can get the people to do the job you still can't train them up overnight
Given this, I find it odd that NATS have this under 30 rule for applicants. 30 is very young to exclude people; pilots have a chance of making it after this age. Reason for saying this is I know someone, 35, who became interested in becoming an ATCO (and, to me, it seemed he would be well-suited to the job) and was disappointed when he looked into the options for people that don't take the NATS or military route.