Currently in my mid teens, I never used to pray regularly. I have now started to pray regularly, so how can I make up for the prayers I have missed?
Prophet Muhammad said:
Whenever one of you misses a Salaah due to sleeping or due to negligence, it is obligatory upon him/her that he/she pray the Salaah when he remembers it because God has metioned, “Perform Salah when you remember me”(Muslim,Hadith 1569)
According to myislam.com you have to make up all the missed prayers because no one knows when the time of death will come, and you should make a clear intention to make up. But, you can simply seek His forgiveness and attend not to repeat again, and then start praying.
I have the same problem. I'm 15 and reached puberty at around 12. So now I pray every Salah a second time. I will do this until I'm 18 since that will make up the 3 years I missed. After that, I'll go back to making 5 fardh salah every day instead of 10. This is working for me, just remember how long it took you to start praying regularly again. The gap should be the amount of time you continue to do this.