Jan, in Excel dates are just sequential integer number starting from 1 which is Jan 01, 1900. Thus Nov 01, 2018 is equal to 43374. For the calculations it doesn't matter in which format dates are presented.
As Vitalie recommended if you'd like to have the age in form like 37.23 years better to use YEARFRAC. If in form like "28 years 11 months and 5 days" - better DATEDIF as in support article.