The jumping thing - just like a horse, there is pain and/or weakness, possibly with the back legs. Start keeping an eye out for ways to help your dog make those mobility transitions with dignity. 
Mine all his life leaped into the back seat of the car with great exuberance from about 4 feet out. Now at age 13 he insists there is no other way, even though his back legs no longer do their part. I tried to transition to a quieter, one step at a time method – oh hell no, he will launch as always, forever and ever amen.
So the way it works is I make sure I’m not holding anything, so I can quickly scoop his butt up and in as he sort of flies through the air as he has always done. Otherwise just the front end launches but the back barely comes off the ground. This move took some awkward trial and error to establish, but we’ve got it down now. Dog maintains he jumped in by himself as always. 
Last year (age 12) I had the vet do a thorough exam, including spine x-rays when the experienced vet thought that was the reason for his weaker back end. We were agreed that we weren’t looking to do much treatment, especially anything invasive. The stress on him probably wouldn’t be worth it if he’s down to just 2 years or so. Just palliative as needed – so far not needed.
I just wanted to know what was going on to keep this dog as happy, able and comfortable as possible, for as long as possible.
The x-rays showed that he had some slight spinal paralysis, which is supposed to be something that aging Labs can have. It will progress (is progressing). Hopefully slowly.
He is happily adapting and just carries on as always with an increasingly modified gait. He has some glaucoma - if he knows, he doesn’t care. He has some paralysis in his throat that is slowing down his eating (I take his collar off to make meal-eating easier). Also apparently typical of older Labs. Barks just fine, though. Still guarding the house (when he doesn’t fall asleep on the job).
The exam gave good info to help me look to the future with this dog. As long as he is happy we carry on. He’s still loosing control of the back legs very marginally, but doesn’t seem bothered – yet. I’ve found some exercises that strengthen his back and help him keep his mobility. But otherwise he was always active and fit, and he’s still pretty strong generally.
Also bought him a better dog bed to keep him comfortable. He sleeps a LOT more. Forget what that cost.
Also re weight management as mentioned above – this dog has kept the same healthy weight all his adult life, within 2-3 lbs either way. He’s never been heavy or skinny. He does get a lot of treats (ever-hungry lab), but they are factored into his total daily nutrition.
Last year he had started having a LOT of anxiety until we started a CBD mix in his daily meals. Now he is back to his energetic happy self and again wakes up to a great day every day. With this dog a mix of hemp with theanine, tryptophan and valerian is better than just hemp. Each individual reacts individually, though.
Honestly the CBD/hemp mix is probably the most important step we’ve taken to make him comfortable, along with the light ‘physical therapy’ (play) that helps him keep his mobility so that he can continue to maintain control over the forests and meadows (aka the backyard).
This dog believes that he is the same dog at 13 that he was at 3. He is out there doing all the things and doesn’t admit that he’s at half-speed. He’s as happy and exuberant as always. He is a model of a great attitude in the face of personal challenges. 
His control over his back legs is gradually decreasing and that is likely to be the thing that finally decides things, if/when he begins to be distressed about it. My guess is that things will be rather different in 6 months - but we will see. This past Christmas might have been his last Christmas, but maybe there will be at least one more. Based on the rate of the changes, sadly I don’t think it will be longer.
But so far he keeps adapting and isn’t bothered. So that’s what I’m doing, too. LOL
This aging dog is precious to the whole extended family. He has a wide support and cheering section. During my parent’s decline this dog lived with them and was an emotional stabilizer and center of attention for them both. He was my aging dad’s best buddy and daily companion, literally until my dad’s last day.
So now I and others are doing our best to do the same for the aging dog. 