There are currently three generations of prolong. The term "generation" has nothing to do with descent or parentage; it refers to the version or variant of prolong available.
First Generation Prolong: May be administered up to the age of about 25; may not be administered pre-puberty. It stops the aging process at the physical equivalent of the very late 30s or early-mid 40s. More effective for some than for others, depending on genetic makeup. Slows physical healing times and/or extends pregnancy periods, etc., without "quick heal" intervention. Hamish Alexander and Alfred Harrington are 1st generation prolong recipients.
Second Generation Prolong: May be administered up to the age of about 21; normally administered about 18 or 20; may not be administered pre-puberty. Stops the aging process at the physical equivalent of the late 20s or early 30s. Much of the differential in effectiveness between recipients has been removed. Slows physical healing times and/or extends pregnancy periods, etc., without "quick heal" intervention, but not by the same degree as 1st generation prolong. Allison Chou Harrington is a 2nd generation prolong recipient.
Third Generation Prolong: May be administered up to the age of about 18-19; normally administered about 16; may be administered pre-puberty but virtually never is. Aside from the case covered in the 3rd sentence of the next paragraph, 3rd generation prolong works equally well for everyone, regardless of genetic makeup. Stops the aging process in the early 20s. Does not slow physical healing times and/or extend pregnancy periods, etc.
In addition to the above, 2nd and 3rd generation prolong are expected to extend the "frozen" aging process by about 20% and 33%, respectively, over 1st generation prolong. (That is, they will both stop the aging process earlier and keep it stopped longer.) Also, for reasons which are still subject to investigation, it does appear that the children of prolong recipients respond more strongly to the same or later generations of the prolong therapies. 3rd generation also plays less havoc with hormone balances and so forth than 1st or 2nd generation prolong.
Honor is, in fact, 3rd generation, despite the error in the earlier book. She is also the daughter of prolong recipients on both sides. She did not receive the treatment until about the time she entered the Academy, which put her through puberty and most of her physical adolescence before it began taking effect.
As for the "jail bait" aspect of her appearance which some people have commented upon, this is a woman who looks to be about 21 or 22 (which gets her out of the "jail bait" category in most jurisdictions). However, remember that she is also half-Chinese. It has always seemed to me that Oriental women appear physically younger (to Western eyes, at least) than Western women do. This is not a value judgment, only a statement of fact (or, at least, opinion), and I cheerfully acknowledge that it may be culture bound. However, one should also remember that the people to whom Honor seems so physically youthful have their own cultural baggage. Alistair McKeon is a 1st or 2nd generation recipient; Hamish Alexander is a 1st generation recipient (and, because of the culture in which he was raised, continues, deep down inside, to carry around a pre-prolong society's views on physical aging); and Andrew LaFollet who, in Flag in Exile, thought of Honor as (I believe) "barely post-adolescent" in appearance is from a culture which (a) did not have prolong at all (prior to the Alliance) and (b) had virtually no ethnic Asians in its population. (And note that, nonetheless, he thought of her as post-adolescent.) The point I'm trying to make is that while Honor does look absurdly young for her actual age, she may not look quite as young as you think (by our standards), because you're seeing her through the eyes of other people with other standards.