For males, for me, the main consideration is size. If the male is too much smaller than the female, she may potentially injure him. I have never personally heard of younger sires producing larger clutches, but maybe someone else will chime in on that.
As for retirement, I usually retire females when their clutch size significantly decreases, suggesting a drop in fertility due to age. I have not had to retire a male due to age in the time I was breeding dragons, but I'm sure they do eventually get too old.
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