The descriptions of grape leaf tea I have seen don't mention a specific age to pick the leaves at - so leaves of most any age are likely to be fine, it probably isn't necessary to only pick those of a certain age, say, the newest leaves or anything. Grape leaves are also edible (stuffed grape leaves, or pickled ones) so you may not need to worry about toxicity, either. However, some sites do mention medicinal value, which means you should be careful about dosages until you're sure about the effects drinking the tea will have on you.
In general, I would expect the tea to be made from mature, medium-sized leaves, since that's the type picked for cooking with (and so most likely to be on hand when someone goes, "what if we made it into tea?"). Older leaves may have less flavor, since that (along with texture) is one of the reasons they're not preferred for cooking with. New leaves might not have enough flavor to be apparent, or might take more effort to get enough of them to make a difference, or smaller size per leaf means an equivalent volume might have an adverse effect on the vines.
Grape leaves are usually harvested late spring to early summer, though if you look at sites dedicated to the harvesting of grape leaves for other culinary purposes, you may find more specific information including harvesting dates or locations for your particular area. The type of grape vine (green, red, wild) may also make a difference to your tea and its effects, though all of them should be nontoxic.