Absolutely. There are a few ways to do this.
In MKTO, create a smartlist for Leads not created in the past 90 days. You'll want to use the "Lead was created" criterion.
For your campaign, use a trigger that looks for when the lead is added to the smartlist.
If you're using SFDC, you can also create a field call "Lead Age" that counts the number of days since the lead has been created (or other date field).
Then, create a MKTO campaign that triggers off when this field is greater than 90 days.