Can I be charged as an felony if I am 18 and a 16 year ok swings at me first?

There are two different issues in this question. 1. Can you even be charged with a felony?    Generally speaking, people 18 years and older can be charged with felonies. However, there must be a basis to charge the felony.  Those are contained within 720 ILCS 5/12-3.05 for Aggravated Battery.  For example, if you fought on the street, or caused great bodily harm there is a charge of aggravated battery for you.  In the typical situation, there is no felony charge for an 18 year old hitting a 16 year old on the basis of age itself.  2. If you are charged with a felony, will you be convicted?   The basic question you ask here is whether your use of force was justified.  The law in this area is located at 720 ILCS 5/7-1.  Significantly, there is an exception for whether self defense was justified for aggressors, located at 720 ILCS 5/7-4.  There are a lot of facts where it is possible that the person who swung first was not the "aggressor", but was in fact defending themselves from the aggressor.   For example, suppose somebody pointed a gun at your head.  They never actually touch you, but you think they are about to shoot you.  In that situation, the law may provide an exception and permit you to swing first.  The answer to your question is, "It depends."  There's a lot of ways a simple battery can be charged as a felony aggravated, and there's a lot of exceptions which permit aggravated battery charges to be filed against somebody who was not the first person to swing.