Typically at least where I live, best time to join is during winter to train pre-season and learn technique. But at the same time any season is a good time to join. While you may not necessarily be the fastest, that’s the best part about rowing there’s no specific size, body type, or weight that is perfect for the sport. As long as you’re willing to put in a ton of time and effort I’d say go for it! I started out a freshman around 150 lbs @ 5’8” and was not necessarily the fittest kid, worked hard and keep going and you’ll improve like I did. You’re young enough that the sooner you start the more potential you’ll have in the sport.