There is no way to ascertain age of a swan by the number of eggs laid or any other way unless you have the actual date of hatching of that bird.

Female birds are programmed with the number of eggs that they will lay at each time and ultimately throughout their life.

We have seen young swans that have laid one egg the first year because they are not as mature in the egg laying process only to have to 8-9 eggs per brood thereafter. So, number of eggs is not a good indicator of age of a swan. The Regal Swan