|Just to be precise, the published script describes him as 10, not under 10, although actors actors younger than 10 have sometimes been cast, most famously in the movie. Harold guesses that Marian is around 26. A 16-year age difference between siblings is hardly unheard of, especially since we don't know if there have been other children in between who may have moved away or died in childhood, hardly unusual and dying in childhood or shortly after birth was even less unusual when the show takes place.
There can be problems in casting, including casting Mrs. Paroos who seem too old to have a 10-year-old child, much less a 7-year-old son, as the role is sometimes cast. The script says she is 40. If Marian is 26, that would mean Mrs. Paroo was around 14 when she had Marian. Which at the time the show takes place would not be all that unusual or even scandalous if she were married, which at that time and place would have been possible.