Grown on generally means that they are older than the babies that are usually seen for sale. A lot of the time snakes are sold when they are old enough to have shed a few times and established a feeding pattern, so a couple of months is common. Grown on would be an older snakes, but I'd still expect it to be less than a year, I don't think there's a strict definition but maybe a couple of the breeders will chip in with their use of the term.
You may also see people refer to older snakes as "hold backs", which are snakes they've kept hold of to see whether they fit into next year's breeding plans, but then they decide to sell them when they firm up their breeding plans.

Make that 1001 posts now.