Developing in an egg is no different than developing in a womb; it doesn't count toward your age. Just sayin'
All other points taken
OR did hiding out in a swamp take centuries off his life? you know how sometimes you look at some people and you just know they had some hard living? yoda.
could just be plot contrivance, hey its been here 50 years, a baby? whaaaaaaaaat? or maybe Yoda and Yaddle were knockin space boots during the prequels and this is their orphan. maybe Yaddle is still chillin out somewhere, then Mando can deliver him and be done with the little rugrat.
They had the whole scene where he's playing with the ship's controls and stuff. I think they just threw the 50 year old thing out there because it's a funny thing to say and have a 50 year old baby even if it doesn't make any real sense.
Except if their aging process is slowed by a factor of 10, that makes Yoda about 90 when he died and LBY would be around 5.
He was just testing Luke at that point though. As soon as he revealed himself, he dropped the act. A character point Rian Johnson ignorantly missed when he made Yoda a giggling twat in The Last Jedi
