But... what does the science actually say...?

Well, like many things, the science on this one is pretty inconclusive. Here is what we do know:

• Pets can provide companionship, which has been shown to reduce isolation and associated depression.
• Pets can provide social support and/or increased social connections which help improve aging outcomes.
• Pets can provide a sense of purpose which can contribute to both better mental and physical health (i.e., walking, caring for something).
• Studies show a link between improved cardiovascular health for dog owners - likely due to increased walking/activity.
• Some studies suggest pet owners have lower rates of asthma and allergies, which can contribute to longevity, but this may be correlational rather than causative.

References

1. Cutt H, Giles-Corti B, Knuiman M, Burke V. Dog ownership, health and physical activity: a critical review of the literature. Health Place. 2007;13(1):261-272.
2. Friedmann E, Thomas SA. Pet ownership, social support, and one-year survival after acute myocardial infarction in the Cardiac Arrhythmia Suppression Trial (CAST). Am J Cardiol. 1995;76(17):1213-1217.
3. Headey B, Grabka MM. Pets and human health in Germany and Australia: National longitudinal results. Soc Indic Res. 2007;80(2):297-311.
4. Levine GN, Allen K, Braun LT, et al. Pet ownership and cardiovascular risk: a scientific statement from the American Heart Association. Circulation. 2013;127(23):2353-2363.
5. McNicholas J, Collis GM. Dogs as catalysts for social interactions: robustness of the effect. Br J Psychol. 2000;91(1):61-70.
6. Wells DL. The state of research on human-animal relations: implications for human health. J Am Soc Prev Med. 2009;24(3):225-232.