It's not surprising for a young dog to eat dirt. By young, I mean from birth to 3 years old, depending on breed Physical maturity doesn't mean a thing. It's MENTAL maturity that's a bit more ambiguous.

Does your dirt have cow or chicken manure in it? My dogs looooved it when we doused our plants with chicken manure. Also, do you have a lot of backyard critters that maybe use your yard as a bathroom? My dogs love all kinds of poop, ranging from dog poop to goose poop. And, yes, dogs can eat dirt if there are nutritional deficiencies in their diet or, more often, it's a nasty habit (to us) with lovely (to them) rewards.