A quotation from a fictional French shepherd

I was reading a translation of the prologue to Marc-Antoine Charpentier's opera Médéé for my French class, Lire Versailles, and I while I was annoyed by the constant (but fashionable and necessary for the time) over-glorification of Louis XIV, I was intrigued by a line given by a shepherd:

In the prime of life,

If you are not flighty,

Tender hearts

Taste but little joy.

The ardour of constant passion

Soon begins to wane.

Do you want happy loves?

Then you must ever change.

In the prime of life,

If you are not flighty,

Tender hearts

Taste but little joy.

It's pretty, in French and English, and it certainly seems true, especially if tender hearts in the prime of life are those that beget those fleeting middle school and high school romances. What about when we're just a little older? When was the prime of life then? 17? 23? What is it now?