"In the Wisdom of Our Fathers (5:22) it says:
"A five-year-old begins Scripture;
a ten-year-old begins Mishnah;
a thirteen-year-old becomes obliged to observe the commandments;
a fifteen-year-old begins the study of Gemara..."
and so on, until the ages of ninety and a hundred.
This Mishnah reflects a similar world-view:
Each age has its own tasks, its own unique possibilities.
So instead of saying to oneself: "Now I'm fifteen; what shall I do when I'm eighteen?" one thinks: "I'm fifteen; what am I supposed to do now?"
This is precisely how righteous persons throughout the generations have been acting.
The focal point of our thinking is not life for the sake of the morrow – not even the morrow of the world to come – but rather life today; "this day – to do them" (Deut. 7:11; see also Eruvin 22a).
What will tomorrow bring?
That's not so important.
What matters now is what is now.
The son of a famous tzaddik was once asked:
What was the most important thing your father ever did?
And he replied:
Whatever he was engaged in at the moment."
From the essay "Tu BiShevat" by Rabbi Adin Steinsaltz