Do people feel emotions less strongly as they grow older? 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).
Deconstructing your question


There is a difference between the ability to experience an emotion and tendency to experience an emotion.
There is a difference between proportion of time that an individual experiences an emotion and the intensity of that emotion.
Obviously, you could take many approaches to answering this question. Emotions can be viewed from various perspectives including physiological, psychological, and behavioural.


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:


Costa et al. 1987 found correlations over 10 years for positive and negative affect of r =.42 and r=.43 respectively. I.e., there is fair amount of stability in individual differences in the experience of emotions.
When Kunzmann et al's review of the literature they cite several large scale studies that showed at the group-level no age effect on negative affect, although a couple of studies suggested their might be a small decline in negative affect with age. These studies varied in the years of life that were of interest, but typically looked at differences between early, middle and older adulthood.
Kunzmann et al also review the evidence for positive affect. The story sounds a little complicated with some studies suggesting a decline in positive affect and others not.
As I've seen before, it seems like cross sectional studies sometimes show bigger effects than longitudinal studies.