If Ace is still alive now, he would be 22. So this means Uta is 21.
Shanks is 39, so that means Uta was born when Shanks was 18.
Believe it or not, some teenagers actually have sex before the age of 18. So Shanks having a kid at 18 does not necessarily mean he slept with a pedo. For all we know, Uta's mother could have been a girl around Shanks' age, too.