Younger teens may still have quite an investment in home life but, as they become more mature, they will increasingly see the world outside of home and family as being where they want to be. This is a normal part of the separation process that teenagers need to go through in order to become independent adults capable of negotiating the world for themselves. They will also learning about relationships with their peer group and will start having more serious boyfriends and girlfriends.
The trick with teenagers is to get the balance right between flexibility and routine. However old and mature your teenager thinks they are, they still need parental boundaries around them. These should be relaxed as they get older. Children of this age will be able to work with most parenting time arrangements as long as they fit in with their busy social lives! Be prepared to be flexible.
If you have had fairly rigid arrangements up to this point, now is the time to think about changing them. Don't be hurt if your teenager wants to spend less time with you. It's not personal, it's just a part of their development into adulthood.
If you find that you are seeing them less, try staying close via email or offer set piece events like a trip away or a day shopping or go-carting as a way of staying involved. Increasingly, it's quality rather than quantity that counts.