Salaah is catogrised into two ways :
Adaa and Qadha
Adaa is the term used which means reading the Salaah at its prescribed time.
Qadha is the term used which means making up for a missed prayer.
So yea Qadha Salaah can be read at anytime...the quicker you make up for it the better because we dont know when we can die and if we die without making up for it we will be questioned regarding it...
hope that helps inshallah !