Why so many 45-49 swimmers?

I have been swimming Masters for two years and am 47 years old. I graduated from high school in 1976 and college in 1980. In South Texas the 45-49 age group has consistently had more swimmers at meets and perhaps the closest competition of any male age group. Why does 45-49 have more swimmers than 40-44 and 50-54, the two neighboring age groups? There are some very fast guys in this age group, who obviously have not taken long breaks (e.g. decades) from swimming. They swim modern breastroke, not legacy breastroke. Are we 45-49 guys just a demographic phenomenum? Our kids are a certain age, we got a bit fat, and decided to get active again? Or was there a swimmer population bubble in the late 70s and early 80s? Is this bubble going to follow me when I age up or does some percentage of swimmers retire at 49? Just an inquiring mind.....