The spark gene in my collection tends to produce sperm plugs a bit earlier than expected. At 4 months old--some a bit younger--the males are producing sperm plugs.

My original sire was only 7 or 8 months old when he first bred. My Lesser Spark male started breeding around 7 months old, too.

I don't think the spark gene is the only one to do this, but it has been my experience that mine all start fairly young. I guess they're horny.