Was there any age that would have been "too old" for a person to enlist?

Further, if a person were too old would they lie about their age in order to enlist? Have you found anyone who did this?

The final question: My Missouri family had three sons, who in 1862, would have been 19, 18, 12. The father was still living, but could have possibly been "two old"; however, it seems that I read somewhere that if a father was deceased then a son would be exempt. I know the 19 year old was enlisted, I assume the 18 year old was enlisted; however, I've always figured the 12 year old was exempt based on his age and IF his father were dead.