So I could be wrong, as I only dabble into historical stuff for fun once in a while, but it seems to me that despite training from the time they could walk and centering their lives around militaristic pursuits, Spartans have kind of a hit-or-miss military record and overall seem kind of average.
Why is that?
Popular culture promulgates a number of myths about Sparta; the "training from the time they could walk and centering their lives around militaristic pursuits" is almost entirely fiction. I'd suggest having a read of /u/Iphikrates' responses to Is the Military "Worship" of the Spartans Really Justified?, and also his (?) fascinating comment on Hoplomachia - for most of their history, Spartans didn't train much more than the average Greek polis did, which was not at all.