He noticed that he continued to age despite reversing time several times, so he is sure that if he dared to reverse time now, it would be an extra 3 years of age that would follow him into the past. So his age at the point he awakens will be 22, not 19.
He noticed that his time and age were flowing forward despite the reversal of time when he saw that his age at the point of awakening was 20 years old, not 19. That was two years ago when he noticed this.
Since the time he noticed that his age doesn't decrease, his age has only increased, and it only increased by the total amount of time that he has lived and experienced, including the time that was lost when he reversed time.
This made him realize that he can't reverse time forever and that he will die one day when he reaches the limit of his lifespan. So it became of the utmost importance for him to increase his lifespan.
