There's never a "right" time as no one knows how long they'll retain their mental and physical capacities.
Trouble is that tends to change very quickly rather than a slow/gradual deterioration for most people.
Once mental or physical deterioration sets in, it gets harder and harder to move, hence why so many people end up "trapped" in unsuitable homes that literally deteriorate around them.
If you're battling with dementia or cancer or some other serious health issue, you've simply not got the headspace to declutter, view other homes, deal with estate agents/solicitors, arrange removal men, arrange tradesmen to update/modernise/change the new home etc.
My parents hit the timing absolutely perfectly. They downsized to a small bungalow the same year that FIL retired, so he had plenty of free time to organise new kitchen and bathroom, redecorate throughout, sort out the garden, replace the garage, etc etc. A couple of years later he was hit by ill health, spent 5 years in and out of hospital and then died. After 5 years of hell, MIL was devastated but at least had a lovely home to live in that didn't need anything doing to it and was "future proof" for her with walk in shower etc. She lived another 15 years! She'd have really struggled if they'd not moved as his illness, NHS screw ups, etc and five years of hell left her devastated and incapable of doing anything to the house or moving etc.