Employee declaration of non competition and non solicitation clause

In a hypothetical situation where somebody decides to leave an accountancy firm where they are employed full time and set up a similar business by taking a few clients with them. Are non competition and non solicitation clauses that may have been signed enforceable even with the clients consent?

Does anyone have any experience or info on this please?

I used to work somewhere, where if I left and setup on my own or joined a rival company, I couldn't approach clients for at least a year. Not sure if it was legal or what, but it was in my contract. I left to do something else, so never came up.
Taking clients when you leave sounds like it could be an issue though.