I dont really know because none of my turtles are agressive, and all but 2 are under 3 years old. However, I was told that eventually, yes it was likely they would become aggressive and unless in a huge pond not be able to be kept together.

BTW, my 3 year old male redbelly, thinks its always mating season, he never stops trying
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1.1.0 YBS
1.3.0 RES
1.0.0 red belly cooter
1.0.0 Fire belly toad
0.0.1 Savannah Monitor
0.0.1 Blackthroat monitor
0.1.0 Leopard Gecko
0.1.0 Mountain Horned dragon
2.1.0 Ball pythons
cats, dog, ferrets, rabbit, rats.