Drawing from photos is a great learning tool and if you take your own photos, you can keep doing it as a professional.
But the sooner you can start drawing from your own photos, or drawing from real life, or your imagination, the better...for several reasons.
Like you, I made sketches of famous people when I was young. But as I grew older, those drawings became meaningless to me. They were learning exercises and nothing more. I actually threw them in the trash not too long ago.
But the drawings I did of family and friends, during that same time period, I kept. Because those sketches DO mean something to me.
I know sketches of family/friends/pets aren't going to get as many clicks as sketches of johnny depp wearing a captain jack costume, but they will mean more to you later, and the sooner you can start taking your own photos/reference images, the sooner you will develop your 'voice/style' as an artist.
Grab a camera, make some costumes, set up a light...and pretend you're making a movie, but instead of filming, just take a set of photos....then pick the best photo and draw it! This gets your family/friends/pets involved and it makes your work 100% original.
Somewhere inside you, there is a unique style and voice shaped by the life experiences that only you have had. Find that voice and develop it!