When serving on jury duty you must be at least 18 years of age so if you're under 18 years of age you will not be summoned for jury duty and if you are accidentally summoned for jury duty when under 18 then you can just show up and they'll excuse you because of age.
However when you're 18 and above you cannot be excused from jury duty just because of your age unless you're 75 years old or older.
Once you reach 75 years old you can show up to jury duty and they'll excuse you for jury duty because of your age and you won't need any other excuses or medical problems to be excused from jury duty.
If you're under 75 then the age alone will not be an excuse from jury duty unless you have other health problems that prevent you from properly serving on the jury duty.