I suspect you will have difficulties finding a very clear consensus on the internet about much of anything. However, I may be able to shed some light. When we were kids, we used to use pine shavings for reptiles and rodents because we were told cedar shavings were bad for small animals. We used pine shavings for years without noticeable problems.

Now that there is so much more information available, there is some better data. An early medical study involving wood bedding and hepatotoxicity found a connection between red cedar, ponderosa pine, and white pine and elevated liver enzymes. Generally speaking, cedar appeared to be worse than pine regarding liver enzyme activity. Another study was able to induce liver damage by housing solely on pine shavings and some found a correlation with an increase in upper respiratory cancers. Both studies were in laboratory mice. For what it is worth, hardwoods like aspen did not show the same levels of hepatotoxicity.

My suspicion is that most animals can live on pine shavings with no observable signs of illness that would cause most owners alarm, but if the study was done, would potentially correlated with a decreased lifespan. In one anecdotal report, one individual who kept rats normally observed them living for 2-3 years, but when he switched one to newspaper, it lived for approximately 6 years. Clearly the "n" value here is not sufficient for any legitimate conclusions, but perhaps it is food for thought.

I am curious now though, who told you aspen was bad / where did you read this information and what was the claim?