What year MH models are too old to consider buying?

There really isn't a hard line not to cross with age. You need to judge each coach individually. Care and feeding by current and former owners is probably the key to the decision to buy or not. Also your skills and ability to perform maintenance should also be a driving factor. Older rigs take a bit more care and feeding to ensure continuous service. Taking a coach in to a RV Service Center for maintenance such as a roof seam reseal will get very expensive very quickly.