Widow
A note or social letter should not be addressed to a married woman - even if she is a widow - as Mrs Donna West. A widow always keeps her husband's name. If her son's wife has the same name, she becomes Mrs Timothy West, Senior but if there is no other in her community with the same name, she simply becomes Mrs West.

Boys - Master / Mister
Until boys turn six or seven they are addressed as "Master". Thereafter they do not have a title until they graduate from high school at approximately eighteen. They then take on the adult title of "Mister."

Ladies - Miss / Mrs.
"Miss" is used to address young ladies from the day they are born until they get married when they will then be addressed as "Mrs". Both the first and last names are used on envelopes. The only time a girl is addressed as "Miss Bates" is on the inner envelope of a wedding invitation.

Messrs
"Messrs." is only used when writing to unmarried brothers. It is not correct to address a father and son.