I don't know about the Walmart near you but the one near me gets really nice Bettas quite often. Thier condition after about a week there, however.....I won't vouch for. I do also prefer to spend my money at my local Ma & Pa non-chain lfs as much as humanly possible.
Most of the Bettas sold at any store can be anywhere from 6 months to 1 yr old. Unfortunately sometimes suppliers will send older breeders, so you often can't possibly know how old the fish you're getting is. Longevity, once in your care, is based alot on water quality and feeding habits. If you're keeping your betta in an unfiltered environment without changing the water but once every couple weeks, as alot of people do...that can certainly shorten his lifespan. Overfeeding and feeding too much protein is another Betta killer. There is alot of great info on Betta care and diet on the web by simply googling "Betta Care".
This site is a good place to read up.
http://www.bettatalk.com/betta_care.htm