What is the age of inclusions found in a rock relative to the rock in which they are found?
inclusions can be older that the rock is which they are found. or younger.
A volcanic dike that is an intrusion into a sedimentary layer must be younger than the sedimentary layer, as the sediment must be laid down before the volcanic dike can intrude or push into or through the sedimentary layer.
If a rock that is not like the rest of the sedimentary layer in which it is found must be older, than the rest of the sedimentary layer. The rock must have been formed earlier and then washed down with the new sediments to have been included into the sedimentary layer.