I personally think that old age & middle age are a state of mind - I have a 39 year old friend who has seemed middle aged since she was about 25! But now she has her first baby she seems younger all of a sudden.
Having children later in life seems to put off middle age; and working later into your 60s / 70s as I know many people do seems to put off old age.
My friends' parents who are retired seem much 'older' in their mindset than my parents & colleagues who are still working.
Also health has a lot to do with how 'old' you feel. I am quite unfit & on lots of meds so get tired quickly & feel 'old' sometimes despite looking quite young for my age. Once I start back at the gym & lose weight I will feel younger again hopefully.
And your social life can make you feel younger - I have a 50 yr old colleague whose social life I envy.