50+ lvl Mage
You could stick at wolfs or unlock leafcats at Ayotha dungeon, amazing job but crappy base. Another good source of exp would be moscovia dungeon though i personally perfer to keep the quest till i get higher in lvl since the quest gives 6mil base exp.
60~70 Job change
Moscovia though again i perfer to save it till a higher lvl for more exp from the quest since multi lvling is disabled here. Siroma's work to and you might want to get the cards for later/now. Geographers work fine to, great exp but make sure you don't attack them when they are grouped since they heal each other. At 70 you can do the new world quest, you might need someone who can kill the mobsters for you. You can lvl at manuk field after this is done, though you'll need an Evil druid or Bathory armor for this against the heathdraining skill they use/and the cast cancel it gives. If you don't have this geo's work fine to.
80~90+ lvl
An amazing way to lvl a sage fast would be going to kiel 2 with a priest/hp (who should be able to tank a bit) or if you are rich/have an awsome friend with imp clips you can duo bradiums with a priest/hp, carefull for the centripede though (your priest needs to be able to tank atleast 1 since you'll have a pretty high chance to ran into those). This is what i've been doing with my BF's sage, though i've seen brave sages/profs solo at bradiums.
Another way to get a sage into a party is 1. Be the partyleader/organize your own there won't be much people who will QQ about your class that way 2. Get yourself in though another a friend xD "hi i'm *insert class which is needed for the party* i'd like to join your party but is it oke if my sage friend joins?" works alot to.
Another piece of advice,
just do something to make yourself usefull, this counts for all classes really. I personally don't care with what classes i party as long as they are doing something for the party, i don't like ordinary leeches in a party when people are just running up and down to save their own asses/exp, every class can find a way to do something to make themselfs usefull. Stick to your party show them you want to help out and most people will open up their arms for you.