Middle age.
It’s a cheerful euphemism for old men to delude themselves that they’re not (yet!) over the hill.
A phrase intended to offer the illusion of youth and vitality that works most of the time… until it doesn’t.
And for someone in this twilight zone who reads ‘Travelling in a Strange Land’ by David Park, it will have a special meaning and resonance that I’d imagine it hard for other readers to find in the poignant reflections and insightful self-commentary about a life filled with its fair share of mistakes and misjudgment.