As in WPM per number of years lived? lol
eg. A 10 year old at 30 WPM avg then, a 60 year old at 60WPM avg?
10 year old is 1/3 WPM per year lived. 60 year old is 1 WPM per year lived.
See how that doesn't quite make sense now?

Though I guess they could add an option to input your age on your profile, and then collect the WPM data together for all users and display it as a bar chart of something?
i.e a bar for each age range. Like 0-10, 11-20 etc and the height of the bar being the average of everyone in that age bracket.
Though even then, that'd only be representative of a fraction of the users of this site who decide to put their age on their profile.

As for the speed people are supposed to type at certain ages, I don't think any requirements exist (unless maybe you work as a typist?)
But being able to type fast has it's advantages depending on what you do from day to day.

Eg. I'm a uni student, so being able to type fast is advantageous for me since I can finish assignments faster.
Someone the same age as me who spends most of their time training for sports competitions say, would benefit less from being able to type at a high WPM.

Well, that's my take anyway lol.

The way I see it is as follows :
The more time you spend learning to type irrespective of your age - the better you get at it.
Although I have noticed that young people are better at changing their 'bad' typing habits as they have not spend lot of time in those bad habits. Older people are slightly slow at learning as they have spend much more time in typing with the 'bad' typing practices and takes much more time to correct the bad practices and then learn the correct typing methods. Irrespective of the age , this is the trend I tend to notice.

I hope my answer gave you a perspective to think on .
Thankyou

What is your theory?
Are you looking to see if there is an apex of age and WPM?
Are you looking to see 'where you should be'?
Are there other factors that play into the mix such as gender, occupation, etc?