I have never heard of this before. My sister bought hers months ago (well before Christmas) and they aren't planning on going to the world til the end of Sept.
Did you speak with a CM at TDS? I haven't heard of this before ... My understanding is that you can buy them and keep them until you're ready to activate it. You should ask a different CM to see what they have to say.
Ask to see a voucher before you buy. If there is no expiration date, then it does not expire. Disney still honors park tickets that were sold 30 years ago, and an AP voucher is nothing more than a "ticket" entitling you to an AP.