It's an option given to you if the app is already in your purchase history for your AppleID. If not, then you have to find a newer device, sign in to it with yo

It's an option given to you if the app is already in your purchase history for your AppleID. If not, then you have to find a newer device, sign in to it with your AppleID, download the app and then return to the old device and hope that a compatible older version is a) available and b) still functional and that the devs haven't turned off any server-side stuff that the older versions relied on.