How old is really up to you. Starting from a rooted clone cuts off about 2 -3 weeks of veg time as in you can veg a clone for 4 weeks and flip with a decent size where as it might take 6-7 weeks from seed to achieve the same size. It also usually takes 7 to 14 days to root a clone soooo 🤷♂️. If I have a good solid rooted clone around 5 to 7" tall, I would consider it about 3 weeks old.
On a side note, if you have a flat white painted wall in your grow space, I would get rid of that mirror. Flat white actually has a higher reflectivity vs a mirror. May sound strange but very true. Download a lux meter app and check out for your self.
Happy growing!!