Just different clothes. Who's going to recognise him? There's no media to spread his face around, after all...a disguise for Aragorn would simply have been the change or name, and different clothing. I'm guessing fancy armour was available sometimes since he was a big soldier for Rohan and Gondor, and a guy called Eagle of the Star would probabaly wear nicer gear than a Ranger's travelstained kit. But nothing complex at all...he didn't need to peel off a latex mask every night before sleeping, or anything like that.
Besides, several decades, even to someone who ages physically as slowly as he does in adulthood, is disguise enough.
Just see photos of your grandparents in their youth, or at the age when your parents were kids or teens. (Makes you think!
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