Hello Xavier,
I have tested the multidropdown, thats nice, but not the solution.
In my database I have only a minumum age and a maximum age. The filter works exact, and that is not working for a customer.
Example: I have a childrens book suitable for 6 to 10 years. (In my database is min age= 6 and max age = 10)
The customer has a kid from 8 years. He would check on min age 7 and max age 9, the book will not show up, because you have to insert the exact min. age and max. age numbers.
And if the customer filters on min age 6 and max age 8 he also do not find the book.
I am still thinking about a right solution and will ask my customer (the webshop owner) her opinion.