"Lo siento" is correct, not "la siento", and it can be used for both female and male, young and old...

No, it is still "lo siento", which literally means "I feel it". Make sure you say it only for more serious things, though, where depth of emotion is important (re: I truly am sorry). For minor things like bumping into another person, "disculpe" and "perdón" are more appropriate.