Unless I miss something...
When I chose 2 teens to make a new look of an young adult / adult sim (and every time is random. I see a good looking face and want to replicate it) MC shows me the menu of Play with genetics, shows no sims and do not close what ever I do. I can only save the game and restart it. I use ver 133 for patch 1.67 (CD)

Not sure why that would be malfunctioning. But the way I get this to work is to first click on the target sim, the one that is meant to pick up the genetics from two donors (parents). That sim has to exist first, even if they are only a baby right now (though it should work on any target sim). I don't believe that PlayWithGenetics is meant to create a new sim.

After selecting the target, PWG should ask you to select the parents from the usual MasterController filter choices.

Is this how you are trying to use it, or something different?

And what I mean is, on the target sim, NRaas > MC > Advanced > Play With Genetics, then select the parents. And then, the screen with the parents on either side and the target in the middle with the pacifier button to re-roll should appear.

( Play With Genetics, then select the parents. And then, the screen with the parents on either side and the target in the middle with the pacifier button) Just choose both parents as teens. What you see? When I click X on the screen before this one with any sim in town, I see as you might see only face pictures. I can not tell which is teen or not. Try and pick 2 teens that you know that they're teens. What you see? And what the hell are you human .... igazor???? You amaze me!!!!(LooLLLL) You answer every here at any time!!!

Try and pick 2 teens that you know that they're teens (I mean as parents)

I'm not in-game at the moment but I'll do this a while later and take some screen shots. I actually can't remember the last time I would have used teens as genetic donors/parents on PWG, so let me try when I can and report back.

When you post on a mod specific Issues message board, which is fine to do, you are going to get the attention of the developers, the management team here (of which I am a member) and a few members who like to check all boards through Recent Updates. Chatterbox gets more views generally and therefore responses from more members. Also a matter of timing, yes my schedule is unusual but I was asleep and then doing other things so away from the wiki for around 10 hours just earlier.

Okay, I was able to re-create this issue in a couple of different worlds. PWG works fine with adults in the genetic donors (parental) slots. With teens, the PWG screen freezes before the slots are populated and (in my case) the cursor keeps spinning. There is no way to dismiss the PWG box. Oddly enough, I can keep playing the game but who wants to play the game with a huge PWG box taking up most of the screen. Trying PWG again with adults once this happens within the same game session didn't help shift anything, the empty box was still stuck.

Not providing a screenshot because it looked exactly like what another player reported here though for them it seemed to have been a one-time thing. And mashing the ESC key as suggested back then did not help.
http://nraas.wikispaces.com/share/view/73490702

I can think of a few temporary workarounds involving aging the teens up to YA, running PWG, and saving the changed target in a different household or in one down in the household bin in Edit Town, but they are all really clumsy and involve replacing the current sim with a new one when finished. So that's probably not what was wanted at all. The other workaround is to just wait until the teens are YAs to do this. How old the child will be by then will depend on the length of your age stages and their current age to begin with.

But maybe our developer will see a different/better way to work through this.

Oh I see, you aren't trying to run this on an actual child. Okay, then my suggested workaround would be to age the teens up to YA, run PWG, and save the resulting sims in households down in the household bin. Then quit without saving so none of this would really have happened. When you reload your game, your sims will be teens again and you can place your saved sims who will still be in the bin (as those are from Library files and not dependent on saves) into your world. Will that work for now?

Unfortunately this is hard coded for YA or above in an EA function which makes it difficult to change. I'd recommend igazor's solution.

The issue, for me at least, is not that I can or I can not transfer the genetics of 2 teen parents to a young adult sim. The issue is how I can get rid of PWG menu (the PWG screen freezes before the slots are populated) if I random choose teens as parents with out restart the game. Is it difficult to create a reset option for PWG except OK and Cancel to unfreeze its screen?