I think it's a great sign he's considering coaching that long, and honestly as long as he can still recruit, and Alabama can still pay assistant coaches well, I'm pretty happy to have him around, and I think he can keep the team in contention.
In terms of when a coach actually might run out of steam, Bear Bryant got his last championships at 65 and 66 and it seems to me he did a bit of hard living. Bobby Bowden got his last championship at 70. Steven Spurrier is 70 now, may be he's finally run into a brick wall, but he's had South Carolina consistently exceeding their historical performance well into his late 60s. Joe Paterno might seem like a good example of someone coaching too late in life, but he really never had consistent performance. You look at any point in his career and you'll find 5, 6, 7, 8 win seasons scattered all over. He had back to back 11+ win seasons in his 80s, and that's actually something he hadn't done since he was in his 50s. I admit he looked old, feeble, and incompetent, but his results really didn't change much.
So, you know... I'm much more worried Nick Saban retires too early than that he retires too late. Bare minimum I think he has a couple more good years in him, but truth is he could easily have 7-10+ more good years left as well.