There are some large name Tea vendors in the united states, which I would encourage people new to tea to check out for pretty standard teas (red/ black teas, and perhaps some oolongs, and if I can't convince them otherwise flavored teas). I will not name those vendors here, because this is not exactly a favorable post for those vendors. The biggest window into the tea world in my eyes comes when you start to realize its not just your (UK nationality) Breakfast, or Earl grey, at the same time you get into loose leaf teas.
Based on what some of these ultra large tea companies carry, I am slightly surprised that so many people stick with their transition to the wide world of tea. With all the health promotions out there, most peoples step away from black/ red tea would be a green tea, which is a lot more "time sensitive" than most black/red teas. Green teas are one of those teas that when slightly out of date can absolutely ruin the whole tea experience, making it exceedingly bitter, and often causes it to lack the sweetness green tea can have (gyokuro, and certain other green teas when carefully aged excepted).
I thought it would be interesting to post my thoughts on how time sensitive certain teas are when being kept.
In decreasing time sensitive order ( lower means stays fresh longer under most conditions).
Med Roast/ oxidized oolongs
Heavier roast/ oxidized oolongs
Not listed: White Tea, or Yellow tea as I do not have enough experience with them.
(*) Japanese greens are an oddball, based on how most of them are packaged they can stay fresh for far longer than Chinese greens, or Korean greens if they remain sealed and stored away from extreme temperatures. But once opened they tend to loose freshness the quickest.