At what age or size do tiger larvae morph?

I have had mine for nearly a year & i got it at 5 inches & its at almost at 8, is there a certain time they morph or they just do it whenever they decide? sorry for the rookie question,

also he used to eat earthworms , feeder fish , crickets , bloodworms, everything , about a few of months ago i gave him a couple of super meal worms as treats , after that first day it wont eat anything but meal worms , i was surprised because the next feeding after the introduction to meal worms eat wouldnt even look or bite the earth worms, fish , anything

i was reading that the shells of the meal worms might cause impaction, even tho when it poops it poops shells , my axys eat earthworms , meal worms fish everything but my tiger is a meal worm addict any help?

ps i have skipped a couple of feedings & let it build an appetite & nothing still wont eat anything but meal worms mad face is for myself introducing it to meal worms

"Usually", tiger larvae metamorphose 2 to 5 months after hatching but up to a year is not unheard of and depends on environmental factors and their influence. You may have an animal that will remain aquatic and gilled as an adult: Caudata Culture Species Entry - Tiger salamander

Regarding mealworms as a sole food source...unfortunate as is any single source food. You may want to use newly molted/shed mealworms to avoid the exoskeleton if mealies are the only thing he will eat. I would continue skipping feedings.....several.... and continue to attempt with other food items especially nigtcrawlers/pieces.