On the Shortness of Life, Section 9

Seneca — The Senator

So there's no doubt about it — the best days are the ones that slip away first. This happens to miserable people, which means busy people. Their minds are still like children when old age arrives. They reach it unprepared and defenseless. They never looked ahead. They stumble into old age suddenly. They never noticed it creeping up on them day by day. Just like conversation or reading or deep thinking can distract travelers — they suddenly find themselves at their destination before they realized they were getting close — that's exactly what happens in life's fast, never-stopping journey. We travel at the same speed whether we're asleep or awake. But busy people never notice they're moving until they reach the end.

Seneca — The Senator Original

There can, then, be no doubt that the best days are those which fly first for wretched, that is, for busy mortals, whose minds are still in their childhood when old age comes upon them, and they reach it unprepared and without arms to combat it. They have never looked forward: they have all of a sudden stumbled upon old age: they never noticed that it was stealing upon them day by day. As conversation, or reading, or deep thought deceives travellers, and they find themselves at their journey's end before they knew that it was drawing near, so in this fast and never-ceasing journey of life, which we make at the same pace whether we are asleep or awake, busy people never notice that they are moving till they are at the end of it.