They reach maturity young, they don't lead short lives.
Though they might lead shorter lives because of Jake. Jake's already middle aged, and he's not much older than Finn, he's gotta be under 20, and their parents both passed away fairly young.
Seeing how Lady's parents are doing, she's probably going to outlive Jake by quite a while. She probably won't outlive her kids, they're only half dog, and dog lifespans don't seem to be THAT short, but still.
I give Jake about 10 more years though, based on how he's aged, and how his parents lived, I can't see him living past 30 in human years.