As far as I know there's no option yet in Qlik Sense to allow only one selection in a field. You can add calculation conditions to a chart object so that it doesn't show anything unless the calculation condition, like =getselectedcount(MyField)=1, is met.
As Gysbert said, you cannot just allow one selection only, but that not the end as you can always achieve this by building an extension. Coming to visual selection, if you have an additional measure you can scatter plot the data with age and any other measure as the axies.
Thanks everyone for your time in answering this. I will look into the radio box extension a little more and see about using that. I was thinking about it a little more last night and I suppose I could just adapt a table to do what I need. I just liked the way the built in text visual displayed, but I wanted some like that which was dynamic. anyway, maybe in a future enhancement. thanks again.