Something

SELECT avg((YEAR( deathdatetr ) - YEAR(birthdatetr)))
FROM people_tbl
WHERE (YEAR(deathdatetr ) - YEAR(birthdatetr)) BETWEEN 18 AND 115 

Want to add an additional filter like:

SELECT avg((YEAR( deathdatetr ) - YEAR(birthdatetr)))
FROM people_tbl
WHERE `tree` LIKE "SMITH" AND (YEAR(deathdatetr ) - YEAR(birthdatetr)) BETWEEN 18 AND 115