No age or height. Bring some safety glasses. If the track is dry you'll need them no matter how high you are. Maybe even a little 5 x 7 Harbor Freight tarp to keep dirt off the kids.
You might want to take head sets for the 5 year old. Sometimes the engine noise can be rather loud for a youngster. Remember , if you go it is a dirt track. I recommend wearing a hat because dust flies. I always sit at least 7 rows up because of little dirt balls that fly off the track. If you go enjoy.
No age limit for grandstands, but I am sure there is an age limit for the pits.
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