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
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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: