All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The use of a brush made of pig hair to paint the floor does not necessitate that the floor is impure.
Pig’s hair is pure according to the Hanafi and Maaliki Schools of jurisprudence, and this is the view chosen by Shaykh Ibn Taymiyyah .
Therefore, there is no problem about this, and there is no harm on you in praying on that floor.
As regards your previous prayers, you did not clarify to us what is exactly the mistake, as it may not require that you make them up.
In any case, if we presume that you used to abandon one of the conditions of prayer, or one of its pillars, then the scholars differed in opinion whether or not you are obliged to make them up. You may act according to this view as a way of removing hardship and avoiding difficulty.
According to the view that stipulates that it is an obligation to make up the prayers, then you are only obliged to make up the prayers from the age of puberty and not from the age of twelve years old.
For more benefit on the signs of puberty, please refer to Fataawa 87018, 83431, 78498 and 90685.
Allah knows best.