You have to decide this based on how good are they with technology. If they can use smart phones, you should assume they are fairly good with technology. If they use feature phone, they can be either not so tech savvy or their motor/cognitive skills do not allow them to use smart phones perfectly.
Also if the product expects user to have some special knowledge or background to use the product, then you can categorize people based on their background knowledge as well. Imagine you are working on the UX of a specialized software like SAS used for econometric analysis. To use the software the user should have good background in Statistics and mathematics. So even if the person is good with technology, he/she may not be able to use it if the user does not know statistics or mathematics. So background knowledge is also equally important just like their level of technology friendliness.