Old school lolita is difficult to pin down stylistically because it isn't a substyle. Age is, of course, the primary factor here: if it was made between 1997 and around 2005, it'll probably look old school. But there's more elements, tangible and intangible, that make a piece old school-able. The materials, construction, accessories, styling, and overall mood of the coordinate all play a key role.
As the presenter for Old School for Newbies at Bay Area Kei's J-Fashion University, I have a duty to share exactly what qualifies as 'old school', at least for my panel's purposes. By no means am I saying I'm a real authority; I've only been wearing lolita for seven years. However, I think developing some clear(ish) guidelines can help people new to old school adopt an eye for the style. Some of this post is adapted from the LiveJournal Lolita Handbook: give it a look if you're unfamiliar or nostalgic.
In other words, I have crowned myself the arbiter of style and taste, and with this post, so can you!