The easiest way to solve your problem is to convert the structure of your data slightly, such that each data point is a single record, rather than having a record per day with multiple data points.
Here’s what you’d need to do to make that happen:
Create a single-select field where the options represent all the age groups you are tracking (ie, the options have the same names as the fields you currently use to track age ranges)
Supposing you have 6 age ranges you are tracking, every day you will make 6 records corresponding to the current date, instead of one — one record for each age range, selected by single-select, for the current day.
With that setup, you’ll be able to set up a graph where you can see your total visits per day aggregated by the age-ranges that make up the total.