The time from birth to two years of age is four hours. From two years of age, our little people age at a rate of one year every four hours.
The oldest age that I am aware of anyone reporting a female giving birth is 54 years, at this time. If Arthur or Barbara has said there is a limit, I haven't seen it.
The readme notes for the .04 update say that a "failsafe" was put in for "refusal to breed" but I have no idea what that means.
Pausing is actually not a good thing to do because they need "down time" when you are not actually playing the game in order for them to get a full night's rest. I would suggest backing up your game file now, so that you can start again from this point if you fail to have a baby or get the adoption event. You can find instructions for doing that in
this post. As dmbson suggested above, if they argue, just keep placing them together until they try. If they fail, wait about 15 minutes and try again. (The time you have to wait is longer depending upon their desire to have children but based on what you posted, 15-20 should be about right for yours.)