I would definitely have the sudden limp checked out. I've been wondering about this question for years as Soro's gotten older. I think it strongly depends on the individual dog. But I have always been on the lookout for any behavior change through aging. For Sor, I noticed him start really aging at around 10 years old.
At 8 years old Sor could easily do 8 mile cross country ski trips multiple times a week with no issues. We were hiking double digits regularly.
At 10 I could still do some light disc, flirtpole, springpole, etc. But I could tell he was getting tired more quickly, and was getting just a little more sloppy.
At 11 yrs old he did a 20+ mile backpacking trip, though I made him carry an empty pack. He did it without any issues, but you could tell it was much harder for him by how quick he was to nap when we stopped hiking for the day. Still, every morning he would play with Brae before doing many more miles.
At 12+ years old now, I am noticing major changes. And really, this year has been the hardest year for him. We are dealing with a lameness issue and Sor can't hike anymore. Now, I've never seen him quit on the trail in the 12 years I've had him. Even today, I am sure if we hike he will come along. He still goes down banks that are too steep for his body to handle. But he is really sore afterwards, and he plods along (I am sure in his mind he still thinks he's flying), and the limp is there. Honestly, I haven't even done a neighborhood walk with him in weeks because of the dip in his step. His mind is still 100%. The crinkle of a chip bag, and he's there. He still knows all his tricks, has great vision, and won't put up with tomfoolery from Brae. But they are flat out not allowed to play anymore. Sor will even play bow to Brae every now and then, but I don't let them play. Brae's just too strong for him and I know Sor will literally get knocked down.
Somewhere around 10 yrs old I noticed Sor's appetite wane. I posted about it, where he wouldn't eat his kibble. I found out it was because he was 3ish pounds heavier than he needed to be. He was 77 or something. He did not look fat at all. But once I had him lose that weight, he was ravenous again. But by and large, Soro is still the same dog. He is goofy, engaging, and ravenous for food. This is why I would recommend further vet analysis for Pepper. Yes, dogs age and slow down. But all of those changes combined seems a little strange to me. When a dog's spirit is down, I figure there must be an internal reason.