I'd say 2 and 3 are still young. 4,5,6 I'd say prime. As for a human being in they're prime I'd say 18-20 I'm 24 and my back feels like I'm 65? Mine are well out of there prime by 5 n 6. As said earlier all terriers different and it really does depend on what work that have done. Only one job for your back plenty a codeine to kill the pain believe me there great pain killers hard to get enough tho ??