You're right in saying that his age might not mean we get a return on Sneijder because there's not that many years you can expect him to play at his peak but there's lots of different aspects to considering. The player himself, what role he feels comfortable/best playing in and his general fitness. And there's the club management of him as well. Both Giggs and the club managed him(self) very well and he still puts in magical performances. I've said before that I can see Young in the Giggs mould which would negate the purpose of buying Sneijder at all and we should buy a more combative midfielder, someone like Javi Martinez who I see that has a good mix of Carrick (being quite tall, passing) and Fletcher and would be one for a long, long time considering he is only 22.
