is to convert the verb reflexive.

"Se necesita esto cuando se es mayor" is an impersonal phrase. It is similar to "uno necesita esto cuando es mayor" or "todo el mundo necesita esto cuando es mayor" or "existe la necesidad de esto cuando uno es mayor" or "esto es necesitado cuando uno es mayor" If I say "yo necesito esto", the subject is "yo"

also when you are speaking of not one person in general. to say "they eat a lot of food in brazil" you would say "en brazil se come mucho comida". They eat=comen but that means they as in people sitting right there with you, when you say they and mean a whole group of people you would use se come and not comen. also, for things like "they say practice makes perfect" you would say "se dice" and not "dicen" because they in this case is not referring to one group of people it is impersonal. hope this makes sense.