If you really need to use it the spelling is l'aîné (or l'Aîné) for a man and l'aînée/l'Aînée for a woman (this was really rarely used).
It should not go into the suffix field, since suffix is at birth and by definition when the older is born, nobody knows there will be a younger.
It could go in the other nicknames fields but ONLY if it is used in primary documents, i.e. a baptism, marriage or burial record where the person appears as parent or witness and is called l'aîné(e).
L'aîné(e) was not the only way to indicate someone was older, l'Ancien is also found, so you must be sure that it was really l'aînée that was used (more frequently it is the younger who is called le Jeune, and no qualifier for the elder).