I would take the 5. Switch on - measure - done. With the 7, you have to pre-select the age EVERY time, otherwise you can't measure. And the age preselection only changes the display colour/fever limit, nothing else.
Besides the differences in design, the functions are different. Since this comment window is a little too small, here would be the manufacturer's link where all models are listed and can be compared: https://www.braunhealthcare.com/ch_de...