At what age did you start planning your retirement and what age do you want to retire?

Well... I enjoy what I am doing now. My job is my interest that pays well enough to save for 12 months of expenses and, pay off my HDB and family expenses with some passive investments. Max up my FRS and looking forward to ERS. The best part is my workplace (2 days WFH) is next building to Fitness First for a happy workout which I enjoy a lot. Probably retire by 65 years old to start my CPF life payout... :)

My plan is to retire before I turn 40. I am already starting to build a passive income stream, and once it stabilizes, I think that will be the time for my retirement

I would say early 30s... though it would be good to start as early as possible. everyone is different. you have to think about at which age you would like to retire and how much you need by then. you can read this article as a reference to retirement and use this retirement planner calculator to help you in your planning

One should start as early as possible but realistically in your 40s. Try not to fully retire, semi retirement is the way to go.