I have had an Ipad one since the first week it came out. Its battery is slowing considerably but I use it to stream radio and music to Apple TV or a bluetooth speaker--so basically my 'radio'--I have to charge it often or keep it on power but otherwise it works fine. I bought an Air when it came out and replaced my ipad Three which went to my husband who had an IPad One since 3 or 4 months after it came out. That one also still in good condition but, of course, can't have firmware upgraded past IOS 5 so it needed upgraded to be useful for him. The Three is like new and he's a happy camper. So, I would say that they will work fine unless they suffer a catastrophic disaster or the battery gives out--and we use ours quite a lot so even though the batteries are finite they are still usable for the very first Ipads. The biggest reason to replace is when they will no longer suit your purposes. I "push" my ipad with a lot of apps, creation, etc plus I wanted light/thinner, more storage, faster processor. The Air suits my purposes and i expect I will keep it several years and my husband will use the ipad Three for several years. When or if there are firmware upgrades that either won't be capable of AND we feel we need the attributes of those new firmwares, we'll consider upgrading either or both of the ipads. With fairly minimal care they seem to have quite long lives, electronically speaking. I'm a long, long LONG time Windows user but even though I haven't switched to Apple for PC I love and use my iPad for the majority of my computing needs now. It does take some investigation of and coordination of apps but being mobile, being able to access many of my files via Dropbox--just a terrific tool.
