Is growing up a gradual process or does it happen in definite moments ?

Both. Definite moments hopefully get processed to see cause/effect and affects behavior.

Both... Slowly by osmosis as you learn the rules, and quickly and often traumatically by events

I guess it depends how soon it affects you and how much control you have over the events occurring in your life as you go.

I feel i had no choice but to grow up quick since the adults in my life were so poor at 'protecting' my brother and i from any of it.

I was already earning a living by 14 as well as a full school day.
I left home 5 days after my last day of school because i got a job a long way from home and didn't return for 18 months.

Definite moments.

Slow and gradual but for some it never happens