It is only people who actually impose that when you are already old, you cannot have a crush on someone anymore. Attraction is something that people feel throughout their lifetime. There are some kids who are drawn towards different people even at an early age. Even those who are already in their golden years can still be attracted to the people they meet. What matters is that people will act accordingly.
For example, it is taboo for a 29-year old man to have a crush on a 7-year old girl. These are things that are not considered normal in society. Having random crushes can be fun though no matter what your age is.