Motorcycle and Scooter appreciation

you can ride one at whatever age, but if you're under 21 (i think), or within 3 years of passing the regular test (i think) it has to be restricted in terms of power, so most modern 600s will be sluggish due to the weight. you can do DAS at 21 (i think) which allows you to ride any bike.

i'm sure someone will clarify exact ages and that.

It's 2 years after your test, not 3. Everything else is correct. If you turn 21 before your 2 years is up you can do accelerated access which basically means you resit your test but instead of using a 125cc like the first time round, you use a bigger bike a la DAS.