My mare is 8 and i'm training her on barrels...So I don't see why not! :)
To loose some of her weight do slow (walk-trot) excersises at first don't be feeding her a BUNCH of grain (I do half a scoop and 2 pads of hay- twice a day) Lunge her often, work on triming her down.
Don't make her afraid of the bit. Use leg and seat pressures a bunch too!
Remember to slow work her first then progress from there!