I'd say the fly by night brigade, that expect every terrier to be an early starter, have passed through and the genuine men with patience and good understanding of a terrier have resurfaced.
Like my mate says, a terriers working career is a marathon not a sprint. Ease them in gradually and you'll have a life time of digging the dog.
It also depends on the man knowing the line well and knowing which things to look for in a terrier and to know when it's mature and ready.
So in answer to your question, I'd say the late starter is becoming more prominent as the men breeding them ha