Fossilworks (the PaleoDB) is a very good reference that often gives good age dates for higher taxa as well as species. The quality does seem to vary taxon to taxon but has been useful enough to be used as a major resource when teaching Paleo Lab. The site occasionally gets loading issues, though, and currently seems to not be loading pages on my end. There are often literature citations as well, but it does depend on the taxon.
There’s even Wikipedia if you want a quick reference. Many taxa indicate an age range (for instance, Arthropoda is denoted as 540 - 0 Ma and Cambrian - Recent at the top of its infobox). Searching through the article may be able to point you to citations for these dates (here including Braun, Chen, & Maas 2007; Yuan et al. 2007; Yuan et al. 2002; Skovsted, Brock, & Patterson 2006; and Betts et al. 2014 seem to be among the most relevant).