i mean omgicantfreakingbelieveit, spring break is gonna be SO sickkk, i'm not gonna lieeeeee....
tEehEe dOn't wOrry SiLLy, i tHInk aGe iS jUsT a nUmBEr
This sing-song quality may have evolved in infants in order to make the parents and other adults involved in cooperative breeding pay attention to the child. As helpless creatures, infants have only their cuteness to wield in getting others to do their bidding. Still, after awhile children stop babbling, and at puberty they stop being cute altogether to their parents. If juvenile traits are retained into adulthood, it is not in order to manipulate child-rearers but to attract mates (or perhaps to join a band of unrelated allies, if they would view someone with too-mature traits as a potential usurper / dictator).
That in turn allows us to see when females (or males) are designed to wander around the mating market. Girls retain the wrinkle-free skin of small children for many years after puberty, but at some point -- I'd guess in the mid-20s -- their bodies draw resources out of skin maintenance. Ditto for maintaining a more juvenile facial appearance, which becomes more mannish. We should expect the same to happen to the fast-and-slow rhythm and the lolly-lolly inflection of their voices.
I did some googling for "vocal inflection" and "age" but didn't find anything right away. It's hard to compare people from different ages because you really want to hear them within the same time and setting, rather than rely on your potentially biased memory, and there aren't that many places where age groups mix these days. One exception is Starbucks, where you find small children, adolescents, young adults, and every decade-group above. During my daily leisure reading break, I hear lots of these people ordering, talking to each other, and yakking into their cell phones. Of course I don't know any of their ages exactly, but there is a pretty consistent decline in vocal cuteness as they hit their mid-20s. You can accurately ballpark their ages because their clothes, accessories, and conversation topics reflect the life of a 20-something professional who's settled into some after-college job.
You'd think the decline would be gradual, but I find it's a pretty sharp drop-off from sing-song to flat metronomic snapping, with women in their later 20s sounding scarcely more feminine than 40-somethings. It's just as rapid as the earlier transition from babbling to no babbling. This suggests that it's not a gradual wear-and-tear process, whereby the parts used for vocal gymnastics just get rustier over time. It's more like a switch is thrown, which means that it's intended by the body's design plan. In this case a female's cuteness is switched off because she should have already found a mate and be in the process of raising a family by her mid-20s. Her body is transitioning into a state built for motherhood, which explains why her voice becomes a lot more nagging and barking. After all, her children aren't going to obey her based on how cute she sounds but rather how stern.
Is there a group that uses cuteness to attract mates and yet rarely plans to or is designed to raise a family? If so, we would predict them to maintain sing-song speech well into their 30s, 40s, and 50s. I don't even need to look up to see who it is when I hear something like:
Ummmmm, can i just get aaaaa.... (be quiet ho, i'm trying to order a drink) okayyy, could you like totally be my new bestest friend and get me a java chip frappucino wiiiiith... yeah, with lots of foam. omigod does that make me sound like a slut? whatever, i mean let the haters hate. oops, i'm like totally tying up the line, aren't i? omigod i love you girl, you're my savior today -- thanksssss!