When I was 42 I had a one-year-old daughter with 2 doting big brothers from my previous marriage, aged 8 and 11 IIRC. I never thought of myself as old.
Now I'm 73 and def old, slowing down. All through my 50s and 60s I was a house afire.
But I have no complaints other than my newly acquired belly and that my former innie became an outie. I look like shit.
But I feel pretty good and I'm still buzzing around town on my bicycle.
So...
Have fun being in your 40s. That decade is awesome.