Complex sentence: It is a congenital condition and is usually not symptomatic until one's later twenties or early thirties, yet there are cases found where Bertolotti's is symptomatic at a much earlier age.
Split into simpler sentences: It is a congenital condition but is not usually symptomatic until one's later twenties or early thirties. However, there are a few cases of Bertolotti's that become symptomatic at a much earlier age.