Yeah, regardless of cases found where December wed Spring, he does have a real situation here ... he does need a reliable DOB for Sarah ... I suppose he has checked to see if there's any records online for churches where she might have be baptized or christened, married, or buried where such bits of info such as age or DOB or DOD etc might be listed?
Also, generally speaking a woman can be fertile from say age 12/13 or so to age maybe 45/50. So she'd be at the least 13 yrs younger than her first born and no more older than say 45 for her last born.
Second thing to look for is documents by her father, mother, sibs, children than mention her. For instance did she have to have a consent form signed by a parent or guardian at marriage? That would mean (usually) she wasn't yet age 21. Depending on the location of course. If she had to have a guardian court-appointed while "young' that also means she wasn't yet 21.