Appropriate model metric for a truncated response variable?
Either RMSE or MAE will give you a sense of accuracy in prediction.  The exact value of any error metric is up to your specific use case and tolerance.

In other words, if you are predicting ages of 5-10 year olds and see a RMSE of 5 then you might think that you have a significant problem.  But if you are predicting ages expecting them to be 40-80 year olds, then a RMSE of 5 wouldn't be a big concern right?