It's probably just a case of typical Jasec knocking some chick up when he was 15... so he became a father at 16... in fact, that could be the way they work. Getting married at 16. It even happens in some cultures on Earth... so hey!
And since Vala would be 40-ish/late thirties around that time... and she was a host for 10 years (more or less... it really can't have been that long), then that pretty much matches up, with a couple of years to spare, too!
If Jasec was 16 when Vala was born, and she's 40 now, add another 10 years for the time she was a host and that would make him 66. Fred Willard is 70, so that almost matches up.
Yes, I took a lot of assumptions, but in my mind, that works.