Putting aside the many mini-transitions of school, the first significant life transition I recall was in my mid-twenties. Suddenly all the people hanging-out and going to parties were younger than me: my own age-cohort was moving on. A couple of years later I was married and starting a career in computer programming.
I don't really recall any transitions in my thirties through to my fifties. I got more senior but somehow we were all 'in it together' - same culture, same pre-occupations, same corporate-speak.
The strange transition to being old has come upon me slowly. Over the last few years I've come to be detached from those 30-50-somethings. It's some visceral thing, like all of life's transitions: nothing you can put your finger on, no qualitative decline in energy or cognition, just a sense that you've departed (or been ejected from) that younger cohort.
Is this the last transition? I don't think so. When I look at people half a generation up from me - folk in their late seventies and beyond (the 'old old' as distinct from the 'young old') - I see another phase change, another transition in my potential future.
Transitions happen to you: they don't emerge from the inside. It's a collective, visceral judgement of a younger cohort - of reclassification and exclusion. It places a burden upon you of mental adjustment, to 'act your age' and to accept that you have to make a psychological transition, to match the social one already imposed upon you.
Ageism? No. I detect the signs of something deep in the human psyche.