Personally I'm partial to Princess myself. Started cruising them in my early 30's, and never really felt out of place age wise. That said, I'd say most cruises even in my mid-late 30's now, I'm probably in the bottom 25% of the age range (but I'd say that's pretty common). Age range can vary quite a bit from cruise to cruise. Caribbean tends to skew somewhat younger, and on my cruise last week, I was probably closer to the median than I'd ever been (but keep in mind, that was a cruise that was during president's day week, and had a ton of kids on it). The Amazon cruise I did I was probably in the bottom 10% of the age group. From what I've seen Celebrity probably trends around the same age groups, RCL maybe slightly younger, but not sure it'd be significant.

October may limit choices somewhat. Cruise lines are keeping their ships in Europe as late as they can these days, although in general the Alaska ships should be back at that point. TA's from Europe seem to run from late August to early December. And of course, October in the Caribbean could lead to dodging hurricanes.

Personally, to me, anything less than a 7 day sailing is a waste. They tend to be on older ships, and you're just not doing enough to make it worth the trip. You might find one or two 5 day trips that at least are vaguely more interesting, but most of the 4 day itineraries are really pretty unimpressive for their destinations.