Hmm, everyone's definition of Old School is pretty different. Old school for a 28 year old is different than Old School for a 15 year old.
I've seen two schools of thought- everything 1985 and earlier, or everything 1995 and earlier. The 1985 group includes everything that "looks old" to us now a days, including Dragon Ball, all the old sentai series, and everything created by go Nagai. The 1995 group seems to be the cusp of that second wave- when Sailor Moon and Escaflowne and such all rolled out in the early nineties- that really accounts for a big group of anime fans who watched the first cartoon network run of anime.
In other news, I'm trying really hard to find the money for this costume... so much has happened, but I don't wanna let Ladyhawk down... T_T
Also, I am so, so excited that there will be Jem cosplayers!!! SOOOO EXCITED!