Remember that ability score modifiers for age are cumulative.
Having a secondary skill indicates that your character has a hobbyist level of competence in the skill. This means you could judge the value of materials and products related to that skill, and may be able to craft rudimentary items related to it. Your character will always be far inferior to anyone who actually practices the profession.
Basically, this is something to ask the DM about during an adventure. “My character has the X skill, can I…”
Characters will never be able to make money from their secondary skill.1)
Characters may have a desire to acquire a secondary skill during the campaign. If the character can convince someone to teach them, the DM can permit them to learn the skill in the same time period indicated for language learning in the Intelligence rules.Ex PCs will be unable to instruct other PCs in a secondary skill unless they have access to the relevant facilities (e.g. an forge for learning blacksmithing, or a ship for learning sailing).