One of the large factors that comes to mind is that kids don't have much money, so it becomes imperative that their first car is not a waste of money. The problem with Spridgets and Spitfires is that unless you buy one that is totally restored, a person will lose money on the deal. Granted, for those of us in the hobby with jobs and other cars, that's not a big deal, but for a younger person, it's more important that they be able to "trade-up" as they get more money.
There are many great starter classics out there with small purchase prices, cheap parts and good
(and increasing) resale. MGB roadsters top the list. I've seen '70-'73 MGBs as low as $2500 for solid, well running and not bad looking cars. There are still some decent TR6s around, and if you can find one, they're great starter classics. TR7s convertibles have no resale, but will in 3-5years (especially injected cars.) Plus they are very underrated in terms of fun potential...plus they're actually really comfortable for even tall guys like me!
Even though Fiat 2000 (injected models 1980 and newer) are wonderful cars, but the parts are too dang expensive. Still, you can buy a perfect one for $5000, and they are really fun.
In terms of American cars, the list is rather endless, but a few 70s models are starting to appreciate. 75-79 Trans Ams (with screaming chicken decals -- black and red are most desirable) are no brainer in appreciation potential. 1974-1982 Corvettes can be had in good shape for under $4500, and '84-'87 Vettes are under $6000, but their appreciation potential is about 15 years off -- but they're fun cars, provided you're under 6 feet tall and can put up with horrible ergonomics and the Atari dash. Another fun and very cheap car is a mid-80s Mustang GT convertible. They tend to be abused, though.