My kids say:
Birth to 30 is young.
31-60 is middle aged.
Over 60 is old.
Over 90 is fucking old.
People are aging more slowly than they did in the past, better information about health now. Look up 55 year old celebrities. These are certainly middle aged people, they aren't young, and most don't look old either. That is how I would define middle age and it's getting longer. You can't get old at 40, you will be old too long.