As far as I'm aware (as in, unless it's changed), SDA's is actually shoulders and chest too, whereas every other race for under 18's is just back plate, elbow and knee
yeh sda is basically knee, helmet, gloves and pressure suit for junior
What kind of riding? My suggestion would be formula the ones, I have a pair on my DH bike and they are great, very powerful and the best I've ever used, never felt any fade with them but I have never really used them on stupidly long descents like in the alps.
And another fact would also be budget, how much you wanting to spend?