"It may seem from the outside that the lama passed away at a very young age, however he would only do this because his passing at a certain time would bring the most benefit. If their coming and going in a short time brings the best to their students, that is what they will do."
If they find it more beneficial to end their life at a young age and take another rebirth then they will just go into clear light and take a new rebirth . Sometimes, high lamas manifest signs of leaving, it is like a wake up call for the students to transform, push themselves harder and put more effort in their dharma practice so that they can move on to the next level. If their passing away brings the best to their students, they will do it.
For a high lama to remain, the students must have tremendous merits to be near this holy being. Out of compassion, the lama or Guru, will absorb the negative karma of his students. The students need to collect merits to be near their Guru and do long life pujas for their Guru asking him to remain with them and teach the dharma. If there is no sign of students wanting to transform it is no point for the Lama to live until a ripe old age when he can do and benefit more in a new incarnation.
It is very sad to see a Guru leaves his students but it is the students who do not create the cause for the Guru to remain with them.