Translation:The young man lives in an old house.
So, what would be the difference between numa and em uma? I have some Brazilian friends and I've never heard they say "em uma" instead of numa, when saying this kind of phrases
In Portugal and I suppose in Brazil as well, it is just easier to say «numa», since we already contract the prepositions with the definite articles, as in «na». I suppose it is the same difference between "it is" and "it's." In both cases, there is one less syllable to pronounce. :) Hope this helps.