I can only say that as an American living in Amsterdam in 2011 I was incredibly shocked by the holiday even though I was already familiar with it. From an American perspective the holiday is incredibly rascist, though I don't think its fair to just take it out of context and apply one's own viewpoint.

Last year the United Nations nonetheless, investigated into the possible racism of the holiday.
http://www.ohchr.org/EN/NewsEvents/Pages/DisplayNews.aspx?NewsID=14013&LangID=E

Generally when I have tried to talk to my Dutch family about the holiday they get very defensive and don't see any harm in the use of "black-face" makeup for the purposes of the holiday. I think the charge kind of sticks a little bit because otherwise the Dutch are considered very tolerant and open-minded.

In general the standard way of celebrating Sinterklaas stops when the child finds out Sinterklaas doesn't actually exist. Usually that's when they're 5 or 6. Easily explained because this is when they start going to school. Through interaction with the older kids some will pick up it's not real. Sometimes parents will continue celebrating for 1 or 2 years just because it's fun and cozy.

Older kids celebrate in a fashion quite similar to secret santa which we call "surprise". (Pronounced the French way (Sur-pree-suh), not the English way.) I think in primary school we did this when we were about 10/11. The teacher writes everyone's name on a piece of paper and puts the pieces of paper in a bag so each kid can draw one name. You're then supposed to get them a present. Normally there's a limit to how much you can spend. Oh, and you're not supposed to just wrap the presents. Instead you have to create some elaborate packaging. I remember in primary school I made a top hat with a fake bottom, so it was extra hard to find the present.

Many companies have there own Sinterklaasparty inwhich they invite the children of the employees. The children will get presents to a certain amount (e.g. €20)

Must companies stop giving presents to children when the child reach the age of 10.

Of course everybody is free to decide to give presents to family members/friends at the older age and even adults sometimes give presents to each other.