I've definitely experienced a decline over the decades and the amount of each load has lessened as well as the time to build back up a moderate amount has increased.
The biggest factor in the amount is the time between ejaculations and I have the best luck spacing them out at least 2-3 days. There does seem to be a point of diminishing returns and anything longer than 4-5+ days will not give me anymore than the average.
There also does seem to be a little bit more with prolonged horniness or foreplay, but that could be just perception.
Unlike in my younger days when I could seemingly cover my wife in a lake of cum from across the room, now I'm mostly a dribbler with usually one good squirt, two on a good day and a weak third one on a really blue moon day.