This is a subjective question depending on many things to me. Personally, everything we've done with our foals leads to the basics of driving - starting with in the first weeks of life...
If you just mean when do you have them ready to hitch, hitch up to cart for the first time, drive with weight in the cart (I'm heavier than some of my 2 yr olds so I haven't driven them with me in a cart!). I personally feel that most minis and Shetlands may not be ready to pull a cart, with a driver, on the road or cross country for any length of time until they are 3 yr olds and that they have to be properly conditioned - meaning build up to the work. I don't have an official arena - so when they are actually pulling me in a cart - they have to be ready physically as well as mentally to handle different terrrain as well as traffic with the cart and my weight.
I have a 3 yr old 1/2 shetland filly - that while she's big enough and has been conditioned by line driving, ponying from a ridden mount, and lounging - she's still not responding to the bit and to whoa the way I want and her mind just isn't there. While she's maturing physically - I don't want to be in the cart while her mind is still trying to grasp what I want her to do. I refuse to hook her at this time... After 60 days of ground driving including being hooked to a drag (my friend did this against my wishes - thinking she was helping me out), she has the summer off right now. I may pick her up later this fall/winter and do more - depends what I'm doing with the others and wether or not I sell her before then. I'd hope that if I sell her, the new owner waits until she's mentally mature to hook, but you don't know...
My first Shetland filly was started with ground driving and w/i 30 days she was hitched to a cart and pulling both my 4 yr old daughter and myself - at about 18 months of age. We had a 1/4 mile driveway with a turn around at the house (Vet Clinic - not where we lived) and at the tobacco barn that we used as a feed/tack room that added some more distance. The round around the barn was well grassed and "spongey-firm", the drive way was hard pack sand. The filly LOVED it. We didn't spend hours doing this, didn't do any cantering at the time and actually didn't do it for very long/many times as during the winter when the weather bad she wasn't worked at all.
These are the only two pics I have of her driving as an 18 month old.
Then in the late spring of 1998 as a 2 yr old - we started her under saddle - that filly would come when my 4 yr old daughter would whistle - you'd see a little head pop up over grass and weeds and here the "little rabbit" would come! Then Sierra'd slip her bridle on, belly across her back and they'd do the "jack rabbit thing" together. Then when tired, Sierra'd slip off her back and lay at her feet (like you see pics of cowboys doin') and sometimes Stuff would even lay down beside her... Stuffy was raised like an orphan as her maiden dam didn't want her - she spent her 1st 20 days in our kitchen at nite and rode to "day care" with my children (my parents' place). When we moved to NC, she was not quite a full yr old - and when we started riding she often was led along with the ridden ponies... So she saw a LOT OF TERRITORY - both on a lead line and while turned loose and allowed to follow on her own - including stream, pond and road crossings.
"Little miss busy body" is 38" at the withers and I'm thinking of hardshipping into AMHR (not show halter quality, but has produced some REALLY NICE foals for us, 2 of whom I may also be hardshipping into AMHR and showing in harness in the next few years) - still going strong at 16 yrs of age. She still enjoys getting out and going - under saddle, loose or ponied and in harness. She doesn't have perfect conformation and we've never "bridled her up" - she travels just fine with her nose out in front of the verticle - but will hold contact, stand quietly and back up on light pressure (oops - the other day she was gaping her mouth instead of giving and dropping into back up mode - need her teeth checked, she didn't get done last year)... For the most part she's strong and can pull either me or my now 18 yr old Sierra w/ or w/o passengers. When we go to Southern Pines and it's been a while since she's been out - she DOES get tired and winded - some of the trails are deep sand and the cart is HARD to pull. We rest as needed and try not to let her get too out of shape - however it's not uncommon for someone to say - o can I drive the one that's broke - and we pull her out even if it's been months since her last hook.
Different harness and driver, but same cart as 1997....