Generally speaking, most breeders sell at about 3 months or large enough to easily handle 1/4" crickets from my understanding (referring to Panthers and Veiled). Kismet was 9 weeks, and so tiny that she couldn't quite handle 1/4" crickets for another 2 weeks.
Really good breeders: 3 months for the standards/4-5 months for Jackson’s and other montanes
Normal: 6 weeks for panthers/veileds. I’ve seen them for sale regularly at this age. Approx. 5 grams
sketchy: under six weeks for veileds or panthers.
I once was sent a month-old veiled which was WAY too young IMO. I'd prefer them to be 3 months, because I feel that's a good standard for them to be healthy enough to be shipped.