'Looking up' a value
I want to lookup a value based on a concatenated field: Age&Sex&Std_name. Age is a problem because it's continuous whereas each Std_name has specific age categories. So, I need to first use Age to find the nearest matching (<) Age_category. I'd then use Age_category in place of Age in the concatenated field in order to find the value in that matching record.
How would I go about finding the nearest matching (<) Age_category?