A year ago, I was the newly certified OW diver, slinging a 13cu pony. Wanting to have the ability to save myself, from the getgo. Even did a mask flood, switch to pony drill at 45', just to make sure I could do it.
A purely recreational diver, can carry a pony right from the start. The trick is, to not include it in any dive planning, gas management. The only function of a pony is, "oh $#it", and it is time to surface. I did from dive 1 as a certified diver. Recently did my 213th dive, and this past summer, I did the SDI Solo cert. Still packing a pony, and still not including it in my gas planning
Recently did a dive where I packed a stage and my pony. The stage btl was to extend my gas so that I could stay with my buddies who have much better consumption than me. The pony was still for the "oh $#it" moments.