EVERY genealogy site I've found parrots Thurston Genealogies. But that source has Daniel born in 1796 The FaG entry for The Ancient Burying Ground in Bradford has her birth as 1798. Since I question their marriage at age 12 and 10, I am currently going through Massachusetts records and cemeteries, primarily in Haverhill, MA. Daniel and Abigail DO appear to have married in 1808 and had four children, Nathaniel Kimball Thurston, my direct ancestor, born in 1813 or 1814.
And, a couple of hours later, I may have solved it thanks to the wonderful site of Massachusetts Vital Records (for Bradford and Haverhill, MA). According to this, the only Abbie Stevens (her name is Stevans, a variant on the site) was born in Haverhill (as TG states), but in 1788. Daniel, still the son of Nathaniel, was born in Bradford in 1786. This would make him 22 and her 20, more likely ages for marriage. It does mean that his mother, as indicated in the records, is Betsey Webster rather than Martha Bridges, and Betsey didn't die childless, after all, in 1790.
Maybe some of the more experienced WikiTree genealogists can confirm that all this seems sound. I will leave the tree as is until I am certain of this. I am very committed to the principles of WikiTree, by the way. It's up to each of us to fight sloppy genealogy!