Child dla , when to apply for pip

Morning, I've just renewed my daughters dla, they received it on the 8th of July, and renewed it on the 12th July, yes 4 days to make decision, it didn't end untill mid October, so very fast, anyway my question, they have given her it untill her 17th birthday, do I apply for pip then when it comes to the end ,,or will I have to apply as she turns 16, or have they given her an extra year while pip is being sorted, due to back logs, just abit confused to her 17th birthday, were I thought it would be her 16th birthday, , as having to apply for pip, thanks in advance