It's because payday is on Wednesday, I guess you get paid monthly, so you're currently spending money that you received last payday, which was presumably around the end of last month. Don't need YNAB to tell you that
It may have been at a week for ages if you have most of your bills set to go out shortly after payday.
It's calculated with the last 10 transactions and it counts the date you spent the money, minus the date you received that money. It doesn't matter how long it's been in a category, it doesn't count that. It just keeps a running count of inflows set to Ready to Assign and sort of knows "This money was received on 29/07, OK, this money was received on 28/08" - that's how it calculates how "old" the money is when it gets spent.
Age of money is totally useless with monthly pay, direct debits, and being skint, because it will just cycle between 1 and 30 for the whole month, every month. It seems to be set up more to deal with twice-monthly pay, which is apparently standard in the US, while direct debit (which they call autopay) is not standard, so bills may be spread throughout the month, and/or people who make a lot more transactions in a month, which I think is more YNAB's target market. I did try to outwit this for a while by noting down the AOM daily in a spreadsheet and then getting it to show me the average per month and a little graph. I just ignore it now, it is mostly useless information. If you get the Toolkit for Chrome, you can see Days of Buffering instead, which can be more useful, though again honestly rarely look at these.