When I was in my late teens up to around 20 I still believed in God and religion. Looking back, largely to please my Mum.
My views changed because my brother was so dismissive about religion so I started to question it myself properly for the first time. I'd taken it for granted after being indoctrinated into Catholicism my whole life.
Once I started questioning and actually thinking about religion (rather than just accepting it as the dull background to my life) I moved fairly rapidly to become an atheist. I've never once doubted or regretted that change. I feel like it was a turning point in my life when I actually started looking around me and questioning everything, and developing as my own person.