After discussion in #7164 I think this is better.
Max tip age was introduced in #5987 to make it possible to run testnet-in-a-box. But associating this behavior with the testnet chain is wrong conceptually, as it is not needed in normal usage. Should aim to make testnet test the software as-is.
Replace it with a (debug) option -maxtipage, which can be specified only in the specific case.