Yes, (pathogen-free) human milk is preferable, even when there is a difference in age.
Keep in mind, however, that as babies grow and mature, their nutritional needs also change. It is therefore more beneficial for donor breastmilk to either come from someone whose baby is around the same age or from someone who pumped when her baby was around that same age.
This study concludes: “Human milk expressed by mothers who have been lactating for >1 year has significantly increased fat and energy contents, compared with milk expressed by women who have been lactating for shorter periods. During prolonged lactation, the fat energy contribution of breast milk to the infant diet might be significant.”