I have four nephews and three nieces. Youngest is 13 and oldest is 22. I've always been fairly generous (or so I thought) and spent usually £20-£50 on each of them for their birthdays/Christmas.
I always prefer to buy them presents and ask what they'd like so if one wanted a pair of trainers for £35, I'd buy them. My only 'rule' has been if they ask for cash then they get £20. I hate giving cash as it feels so lazy but they want it the older they get.
I'm hoping to start a family next year and I think it's time to consider reducing what I spend. I am considering stop buying for the ones older than 21 (one is 21 this year so she'll get about £50 spent as it's a special birthday). And perhaps reducing the limit to £10-£20. Is this fair?