Well, in my recent experience w/ my two tiger oscars. I was worried about them not getting along and such, and had the EXACT SAME question. Well, mine, one at like 7-8ish in, and one 10in. The littler one was purchased, and the larger was adopted, so I have no idea on age. At first they weren't very friendly, chasing, competing for food and such. Then in the past few weeks or so. They have started hanging out together ALL THE TIME, rubbing on each other, wiggling next to each other, and such stuff. And unless oscars have some kind of weird, territorial ritual that involves rubbing, an obsessiveness of being together, and protecting each other, then I'd have to say they've paired up. The dedication is surprising to me. For instance, the female has a little cave she likes, that only she can fit into, and he's tried so hard to be w/ her that he damaged himself a little trying to jam his way in. No one really answered my question when I asked, so I thought i'd just tell you my experience. I got: "It all depends on the oscar". So for mine at least, they showed their "love", at around 7in(it's hard to measure curious fish). That's the best I can help you with! Also, an odd observation... my female has become much more active and appears to be growing faster since she's paired up. They are very fun and interesting fish. They are surprisingly smart! Well, good luck, and hopefully you'll have a good time with your fish.