Aliens from the planet Zog have activated a ray which will surround the entire earth and make all human beings the same age. (Let's not worry about why: maybe they want to cripple humanity - in which case, replace Zog with Skaro - or maybe they think they're doing us a favour by unifying us.)
They just need to enter a fixed age $X$ into the control panel, and then the ray will take effect. Everyone younger than $X$ will find their bodies ageing instantaneously, while their minds stay the same; everyone older than $X$ will regress to exactly the same state - in body, mind, and experience - as they were when they were aged $X$.
The effects are permanent; the ray envelops the earth forever and continues to affect everyone, kicking in roughly once a day (think of it like updates automatically running on your computer). The human race effectively becomes immortal, their bodies staying at age $X$ for ever. Babies' bodies age to those of $X$-year-old people within the first day of their lives. Anyone who was older than $X$ before the ray has their short-term memory completely screwed, forgetting each day once it's happened, and the same happens to the rest once enough time has passed that they would be aged $X$ even if not for the ray. (In other words, nobody can ever have more than ($X$ plus one day)'s worth of experience.) People can still die in all the same ways they could before, except old age.
The aliens allow us to choose the value of $X$. It can be any age from that of the youngest person in the world to the oldest.
What value of $X$ would be best for humanity, in the long run? Obviously there are pros and cons to either a larger or a smaller value of $X$, but make the best case you can for a ball-park figure for the optimal $X$.
What will be the effects on society of the change? How will it affect interpersonal relationships, bearing children, the increase of human knowledge, etc.?
(Please check the tags - I don't have much experience on this site, and am not sure if they're appropriate for this question.)