It sounds like the original poster was referring to the
Game Boy's Super Mario Land, and not the
NES' Super Mario Bros. But in either case, I was of that certain age were I played both games on their original consoles back in the day! Ah, fond nostalgic memories!
I must say though, that I preferred the sequels to those two games more so than their originals. The
Super Mario Land sequels in particular were re-vitalized by shifting the villain from the forgettable, one-time Tatanga alien to the highly amusing, greed-driven "Bizzaro Mario" named Wario!
I must also confess that I too never finished the
NES' Super Mario Bros during my adolescence. Thankfully, none of my young peers knew of my dark, disgraceful shame!
Much later, in my adulthood,
Super Mario Bros was one of several
NES games that I nostalgically revisited--determined to finally finish them and achieve a delayed sense of accomplishment! I owed it to my young self!
I had no trouble finishing any of the other Mario games back when they were relatively new, including the
Game Boy's Super Mario Land.