В конце концов, lit. "in the end of ends". Sometiems very expressive, especialy in the end of a phrase... sometimes not:-) 1. finally 2. after all. Here just 'after all'. мне уже сорок лет, - I'm already 40 --- here the author could insert the dash "–" if he wished. Because, something like "it is...", "which is..." is omitted. Or he coud add "Это...". Cf. aslo similar "Арно, француз, который задал этот вопрос, живёт в Париже". --- возраст, достигнув которого... lit = "the·age, having·reached which..." (upon reaching which.) от авантюр пора по возможности уклоняться. it·is·time·to to·evade aventures, when possible.

The meaning of this sentence is that when you're 40 it's too late to take risks and take part in some questionable and contorversial activities.

You're right: Finally, I'm 40 years old, in this age you should avoid the adventures

В конце концов, мне уже сорок лет, возраст, достигнув которого от авантюр пора по возможности уклоняться. Literally - Eventually, I am 40 now (already), this is the age, reaching which it is a time to retire from adventures as possible.

Yes, you understand it right. I would say, it is an advice to avoid the adventures if possible.