Can a 401(k) plan be written with a tiered match formula based on years of service? For example: 50% on the first 5% deferred for the first 5 years of service and the match would increase to 100% on the first 5% deferred after 5 years of service.

The clients current TPA is telling them that they could write this into an individually designed document but could not in their prototype. I would have thought that you could not do this in any document.

Seems to me that the 5 year of service "requirement" to get the second tier of the match would violate the minimum age and service requirements for a qualified plan. What am I missing? Can a match be written this way?

This is possible. But you need to have the second tiered match pass BRF testing under the 401(a)(4) regulations and you need, as always, to pass the ACP test.