Debe mover la cláusula where dentro de la consulta como
SELECT pid,
name,
age,
team,
rank
FROM (
SELECT pid, name, age, team,
@curRank := IF(@prevRank = age, @curRank, @incRank) AS rank,
@incRank := @incRank + 1,
@prevRank := age
FROM players p, (SELECT @curRank :=0, @prevRank := NULL, @incRank := 1) r
WHERE team='aa'
ORDER BY age DESC
) s
