Lord Narasimha offers Moksha to Prahlada but Prahlada expresses his concern towards other virtuous people who depend on him. He asks God "how can I leave all these poor people and go to Moksha alone?".
Below is the shloka from Bhagavata saptama skanda Prahlada-kruta Narasimha stotra (9.44)
After listening to his stotra God promises that he would live for one manvantara as king of daityas and then attain Moksha.
This happened in the first manvantara (Swayambhu) so he must have already attained Moksha, but hey wait!
People who have finished their time in this world have to wait for Lord Brahma to take them inside Vaikuntha during Pralaya. Till then they would be living in special places called "Amukta Vaikuntha". I am not able to get exact references for this online, will edit if I get elsewhere.
Dvaita followers believe that Prahlada reincarnated as Sri Vyasaraja teertha swamiji and Sri Raghavendra teertha swamiji