When did things start going downhill?

I have an 8 year old basset. She has been so active all her life…from living on a farm from 2018-2021, to running/walking with me when she was 5 years old, to hiking 5 km no problem (just recently). But lately, I’ve noticed some changes. A few years back she started getting basset bumps…which have really increased in size lately (we’re getting them removed next week). The bumps are large but she doesn’t flinch when we touch them. The vet has observed them and said they don’t seem like they could be cancer…but she’s also not acting like her normal self. She seems sad, depressed, and mopey-er than usual. She lies at the back door but doesn’t want to go outside no matter how often I open the door for her. She’s still excited about her walks but isn’t moving as quickly or gingerly as she used to. Is this just regular aging stuff??? Or should I be concerned with her change in behaviour? When did your basset start getting “old”? And when did you decide it was time to let go? (I know she’s not there yet, but I’m just curious)