Answer: Never lie to anyone--it will only ruin your reputation.
Step-by-step explanation:
Mark Twain in this excerpt is telling the youth not to lie because they would likely be caught and once caught, it would be very hard for people to completely trust them again because their reputation would be ruined.
The last sentence infers that the youth should not lie because they are not yet capable of telling good lies. One can take this to mean that Twain did not mind adults lying but other statements by him seem to support a stance that one should avoid lying completely.