Yes it is possible to alert on snapshot age, as well as size. Veeam introduced new analysis monitors in version 6 of the MP. These allow metrics from different data points to be joined in wmi and displayed in the same alert or state view. The analysis monitor you are looking for is the Veeam VMware: Virtual Machine Snapshot Usage Analysis monitor. This allows you to configure thresholds for both size and age for snapshots and configure alerting etc for your desired group of VM's. A nice benefit is that when your alert comes through it will display the age of the snapshot and also its size in the same alert text.
Hope that helps