score:8
Your passport qualifies to add more pages if you prefer. It costs a little less in the short run, but you will have to mail your passport with the application form.
Although I was not able to find any info specifically on the State Dept website about this topic, any restrictions on how early you can renew are conspicuously absent.
Information on various US embassy pages from other countries also suggests that you can renew at any time:
From http://brazil.usembassy.gov/service/frequently-asked-questions.html:
How soon should I renew my passport before it expires?
You may apply to renew your U.S. passport at any time.
From: http://london.usembassy.gov/cons_new/faqs/faq_passports.html:
How soon should I renew my passport before it expires?
You may renew your passport at any time before or after it expires.
Upvote:4
There is no restriction on how early you renew your passport - it costs them just the same since you're paying for it anyway - it just doesn't make sense to, obviously, if you have lots of pages. However in your case you may indeed want to.
From the Bureau of Consular Affairs: How to Renew Your US Passport
And if you wish to find your closest location to do so, if you're not going to be mailing it in: