Nothing wrong with what you are proposing. A lot of candidates are a little older. However, I was a TKI (& DCTKI) and a UK ATO and one young lad came through the doors aged 16, he had the financial backing and he stuck at it, giving 110% effort, staying behind after lessons and asking plenty of questions. All 14 ATPL subjects passed, first time with a good average. He passed all his flight checks first time and went on to become a FI whilst waiting for his first Airline position to come along. He worked hard at it and is now gainfully employed as a first officer with a good airline and only just 20. All being well, he will be one of the youngest qualified ATPL's around aged 21.
So it's certainly achievable.