Eat My BB:
You can only pass your HGV licence under the age of 21 if you are in the forces.Have a look here.
You can drive HGVs at the age of 18 if you do the Driver Initial CPC.
It’s a weird kind of EU rule, they raised the age limit and at the same time lowered it to 18 for anyone who does the DCPC, as the DCPC is required for anyone who wishes to drive commercially anyway the 21 age limit won’t affect many people.