I believe to drive a class 1 HGV (over 7.5 tonne) you have to be over 21 although it may be difficult to get a job at that age as most insurances of these HGV's only cover over 25's. Unless it's changed I think you can drive class 2 (up to 7.5 tonne)at 18.
Cat C1 3500kg to 7500kg - age 18
Cat C (HGV2) over 7500kg - age 21
Cat C+E (HGV1) over 7500kg with trailer over 750kg - age 21
You must have Cat C already before you can take Cat C+E however you do not need to have Cat C1 before you take Cat C.
This is from my son who is an HGV driver .