What is the name of your state (only U.S. law)? Maryland
I have been married almost 20 years. If my husband and I divorce, am I entitled to any of his retirement fund when he is old enough to access it?
Was the account in existence before you were married or opened during your marriage?
Essentially, you will be entitled to 1/2 of the marital equity unless the account is separate property (that is, existed before the marriage and has not been added to during the marriage).
In most cases, you do not have to wait until he is old enough to access it. You would get a QDRO which would distribute your share to you at the time of the divorce.
IMPORTANT: do not take the distribution in your name. Rather, open a new IRA and have the money transferred directly from his account to the IRA so that you don't have to pay taxes and penalty on the withdrawal.