What age is a tricycle appropriate for?

It depends on the design. Convertible push-trikes (newyoo, BABY JOY, Radio Flyer Stroll 'N Trike) advertise use from roughly 9-12 months in parent-steered mode, while classic pedal trikes like the Radio Flyer Red Rider are aimed at 2.5-5 years. Community threads on r/toddlers repeatedly note that very young toddlers may only ride a few minutes at a time at first, so the parent-push modes get the most early use.