The old set bonuses are currently disabled and we don't know for how long.
I've made thread about this on official wow forums too : http://eu.battle.net/forums/en/wow/topic/17612933365
But I'll copy pasta my thread here too for your pleasure
"Hello!
I've recently discovered that old set bonuses are not active even if I am at the level they were intended for originally. I am lvl60 twink who wishes to complete old content with the old gear, but the removal of old set bonuses kinda defeats the purpose of getting that old gear for us twinks.
Please reconsider this legacy set bonus thing. There's 0 harm for us lvl60s to have the set bonuses.
Same would go for 60-90 twinks aswell.
Why not just have the set bonuses disable when you go pass lvl60 and so on? Or if this change was because people were abusing old gear in timewalking, then why not just disable the set bonuses while in timewalking dungeons?
Thanks for reading."
TL;DR : Old gear set bonuses are disabled and I think it would be fair to enable them for players who are at the level which the gear was designed for originally.