StarTrek.com puts Amanda's birth at 2210, which, if true, places her around 19 or even 18 when Spock is conceived (he is born in early January 2230 according to Star Trek Beyond), with a Vulcan noble in his mid-60s. We also know that she is his second wife or perhaps even a mistress at the time, and their relationship on the whole is scandalous (although this is likely due to the interspecies nature of it).
Non-canon usually places her birth about 8 to 10 years earlier, which is a little more palatable to my 21st century eyes. I mean, I don't care how enlightened people are in the Star Trekverse, it doesn’t seem right for a teenager to be starting a family before she ever had the chance to begin a professional career (of course, yes, she can and did both).
Logically, though, thinking as a Vulcan, it would make sense to "mate" with a young Human woman, if you are inclined to engage in such a relationship, in order to maximize the marriage time between the two. The ~46 year difference between them means that, conceivably, Amanda could live to a ripe old age (which she may have), and Sarek could pass away relatively young (which he, ultimately, did not) without worrying about decades of separation and widowness. Of course, I'd rather their relationship be a little more spontaneous/natural rather than think about Sarek trolling for 18-year-olds.
