As we age, the way we experience and express emotions changes. This is because of a number of factors, including: changes in our body, differing types of experiences at different ages, improved emotion regulation strategies, and increasing experience of dealing with affective situations. However, work in psychology points to differing findings on how emotions, typically valence and happiness, changes with age. Psychologists measure happiness and well-being through questionnaires, which can have biases and result in limited data. Thus corpus analyses can provide useful complementary insights. We compile and release a large dataset of social media posts annotated with the age of the author at the time of posting. We refer to it as AgeCorpus. Using this dataset, we apply simple and interpretable methods to explore research questions pertaining to how social media posts, especially emotion expression through these posts, varies by age groups. Analyzing the emotions expressed in the posts, we find that the average valence increases until the middle ages, and then decreases; arousal decreases (Reddit)/plateaus with age (Twitter); and dominance follows the inverted U-shape (Reddit)/increases with age (Twitter). For categorical emotions, we find they follow the inverted U-shape on Reddit and increase in intensity with age on Twitter. We hope our dataset enables further research into age related phenomenon, such as well-being and language use.