Yes there was another thread earlier about what age people consider elderly.
Outlook, lifestyle and health obviously vary on an individual level but generally I would say that:-
18-39 is young. At this stage people are having children, not always decided on career, little sign of ageing in the average person.
40-59 is middle aged. Children growing up, career minded people will be moving up the ladder, moving up the property ladder for those that own properties, having to work harder to stay fit to prevent weight issues.
60-79 is senior. Children have flown the nest, grandchildren on the scene, slowing down career wise. Most people at this age are still active, enjoying travelling and time for hobbies.
80 + is elderly. Health issues becoming more common, the number of friends/acquaintances that you know from school is diminishing. Not all doom and gloom though! My mum is is in this age bracket and until the pandemic had an active social life and liked to travel. The pandemic has affected her quite badly and she is quite low and depressed which I've not seen in her before though.