In conclusion... If it is a milestone in your mind and in the mind of the birthday person... you need to bring it just a little bit.
A big DO and a big DON'T for your thoughts.
DO: make a memory. If they have reached a milestone (whatever it may be) they will have memories or events that they will want to remember. If you have shared a memory with this person... remind them with a photo or if you are creative, something else. If you haven't, give them something to store the memories: a photo album/ frame. Alternatively, give them the opportunity to create a new memory. Gift vouchers for a holiday or day spa are perfect. Take your mate out on the town and celebrate in style. Decorate their bedroom or desk at work with something significant from their life. Have Fun!
DON'T: make an assumption about the age of your recipient. Let me explain. If the birthday person is turning 40, don't assume that they prefer a hot bath and a wine on a Friday night. Once there was a teacher in my teaching team who celebrated her 40th birthday by attending a Kylie Minogue concert with a group of girlfriends. They followed that with a pub crawl and ended the night in her family room drunk out of their heads singing "Living on a Prayer" with the Karaoke Machine. Her husband came downstairs and told them to settle down and they giggled like school girls until the early hours of the morning.