I don't think that whether a lady was 21 or not made much difference to the propriety of either giving or receiving (though her being "out" or not might); light semi-meaningless flattery and flowers could be offered (on appropriate occasions) without occasioning much comment, or giving something not very personal like game (such as venison) to her whole family in general is always proper. What is most likely to be compromising and improper is objects which are both valuable and personal (such as a chain which won't go through your brother's cross ), especially when given without any fig-leaf of family relationship (between relatives of some kind) or long-standing family friendship.