I'm specifically interested in whether or not their maximum ability to feel an emotion decreases as they grow older (so by that I'm meaning - let's ignore the effects of tolerance induced by life experience for now).
closed as not a real question by Ben Brocka, Josh Gitlin♦, Steven Jeuris♦, Matt Rockwell, Jeff Jan 23 '12 at 23:35
It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, see the FAQ.
Deconstructing your question
Some empirical evidence
I did a quick search on Google Scholar for 'panas longitudinal study'
PANAS (see Crawford and Henry 2004, PDF for a a review) is a commonly used self-report questionnaire that aims to measure the frequency with which various positive and negative emotions have been experienced over a given time frame.
I found this interesting article by Kunzmann, Little, and Smith 2000, PDF called "Is age-related stability of subjective well-being a paradox? Cross-sectional and longitudinal evidence from the Berlin Aging Study".
The opening section of the article reviews the empirical literature regarding stability and age related trends in self-reported positive and negative affect. The following are a few interesting points from the article: