Question

how did King Arthur turn Victoria at the end​

Answer 1

The Historia Regum Britanniae explains that when Merlin died he was "400 Summers his Lord" (His Lord would be Arthur and Arthur died in his 40's to 60's). So we can assume that Merlin was around 440-460 years old. However his character was based on Myrddin Wyllt. Myrddin died at the age of 39.