Especially as a younger female engineer, I consciously try to avoid looking younger since people consistently underestimate my technical abilities. I'm also average height for a woman but typically the shortest one on my team, so if I show up to work wearing a T-shirt, jeans, and converse I can look even more juvenile.
Here's an example of how changes to your casual wardrobe can make a huge difference in how old you look: http://www.extrapetite.com/2010/05/reader-request-how-to-loo...