I work for one of the largest tyre retailers in the uk .
We have a 5 yr age policy .
What you will find is the common sizes have a fairly high turnover so better chance to find newer dot codes oddball sizes will either be older or brand new as have to be made for the order or will be from a previous molded batch.
If tyres have been racked and stored correctly should be no problem with them.
Like has already been said i would be more concerned about a set of tyres that are fitted and sit inflated on the damp ground for months at a time causes premature cracking and splitting of the contact point