Long story, short: I started playing at age 10 and was fortunate to have a bit of ability but also spent hours practicing and playing, so I got to scratch before I was 18. I even built my own crude short game area in our yard. Then when I was 20-21 I quit golf due to other interests and obligations. When I finally picked the game back up some 15 years later I was no longer scratch...far from it! (I remember my first round back was like 120-something!
) So to get back to single digit I played and practiced a lot (not like I could as a kid of course), especially the short game. I took a few lessons to get back some of the fundamentals. I never made it quite back to scratch but I don't care to be either as I rarely practice anymore nor do I have the desire to. I hover between a 4-7 handicap these days and I'll still play in some competitive events like our club championship or our match play events but for me golf is now more about having fun with friends more than anything.
Good luck on your journey!
PS...I agree with the posts above that say the lower you get the harder it becomes to go lower. Getting back to single digits was relatively easy but that's when it gets harder to shave off the strokes...especially, I believe, the older you are.