If you want a two-digit number, the easiest way would be to use one chapter to let the reader choose the tens' place and another chapter to let the reader choose the ones' place.
If you want to use this for age, make sure that you end up with a number of 18 or above. (If it isn't done like that, the respective character will be considered underage.)
Two ways of doing that:
1. Let the reader choose the tens' place first. If it is 10, disable the options of 0 to 7 when choosing the ones' place.
2. Let the reader choose any number and then apply a minimum value.
This might make things easier if you want to use a system where you can incrementally change the value. E.g. with +10, -10, +5, -5, +1, -1
To apply the minimum value, add the following score changes before the reader can leave the age choice section.
variable type: numerical (4 score changes)
Code:
age - 100 000
age - 18
age + 100 000
age + 18
variable type: progress (2 score changes)
(This approach can also be used to apply other minimum or maximum values to variables.)
2. Create a following chapter where the title is "I'm 20 years old," and have an age variable set to 20 in the chapter. Have the story continue from that chapter.
3. Do the same for 30 and 40, and have link chapters from each which link to the continuation from the 20 chapter.
If you want to use the "20" chapter for the continuation, you need to take into account that score changes of both the link and destination chapter will be applied one after another.
So you should use the following setup:
Choose 20 (directly accessible and link chapter: age+20 (age=20 won't work here)
Choose 30 (link chapter): age+10 or age=10
Choose 40 (link chapter): age+20 or age=20
Choose 50 (link chapter): age+30 or age=30