What differences are there between using 'at' and 'of' as prepositions before 'the legal age' and possibility other age-related noun phrases? I have encounter both constructions, though 'of' pops up more frequently in formal contexts. Why is that the case? Another pattern I noticed is that 'of' always immediately follows a noun, whilst 'at' enjoys greater flexibility. Bit what about the example below? Which one is the better fit and why?

• He is now [at the legal/of the legal] age to go off to war.

What differences are there between using 'at' and 'of' as prepositions before 'legal age' and other age-related noun phrases? I have encounter both constructions, though 'of' pops up more frequently in formal contexts. Why is that the case? Another pattern I noticed is that 'of' always immediately follows a noun, whilst 'at' enjoys greater flexibility. Bit what about the example below? Which one is the better fit and why?

• He is now [at (the) legal/of legal] age to go off to war.