My friend's Golden Retriever is 13 and although it's been apparent all her life that she had either dodgy hips or back (imagine one of those paintings of a horse done in the days before people understood how galloping horses move and painted them like rocking horses, well that was Ruby at speed
) she didn't show any symptoms of pain until she was about 10 and then only mildly for some time before suddenly turning into an old lady and developing some other niggly little problems too. So I think I understand what your vet is saying.
Ruby seems to have adapted fairly well after an initial period where she really lost interest in most things except food and became very focussed on my friend to the exclusion of everyone else. She's always been a bit of a one person dog, but it became even more apparent. She is on quite strong pain meds now, as well as herbal and other supplements, but the nice thing is that she has regained her interest in life. Her exercise is limited and slow, but she enjoys being out either just with her mum, and when William, Louis and I are there too, or her other friend Oscar. She loves travelling in the car, although has to be lifted in and out, even if she's only out of the car for a 10 minute stroll round a garden centre or sitting around watching the world go by while her mum has a coffee.
Certain things distress her that didn't used to, such as a lot of noise, or her mum being out all day even though other family members are at home. So I think she has less confidence, but overall she's got her mojo back. It's just a lot slower than before
I hope Ben can adjust to his senior status too