Take a look at the manual I recommended, it will answer a lot of your questions. For example, Age (Table 7-3 Eligibility Requirements for Direct Commission Officer Programs) specifically states: Maximum 34, as of 30 SEP of FY in which selection panel convenes. Given that you would be 35 as of 30 SEP in FY21 (for DCO 21-2) - I would double check with your recruiter that an age waiver isn't required.
There is no set time frame to find out results, but if you look at historical averages it's around 30-45 days post the board convening and then being subsequently signed off by the DHS Secretary.
Following selection the detailer will reach out a couple months later, prior to commissioning, for duty location preferences. Not everything is available, however if you have a unique circumstance (Dual Military, Exceptional Family Member Program, etc.) there is some flexibility. The last board (pure-RW candidates) received 11 choices between 60/65. To my understanding you can rank them however you like, i.e. platform or location. You'll get one of them based on the needs of the service, personal preference, etc.
If you're unsure of the duty stations and their platforms, Chuck Hill's Blog includes the USNI Aviation Order of Battle from 2018. It does a good (albeit dated) breakdown of each platform and air station. (https://chuckhillscgblog.net/2018/09/08/2018-aviation-order-of-battle-usni/). You're in good shape either way as the MH-65E comes online (Glass Cockpit) and the MH-60T Sustainment Program is acquiring 18 new hulls over FY19-20.
Each DCA is required to attend the T-Course at ATC. I haven't heard of prior H-60 pilots receiving preferential treatment for platform one way or the other. Hope that answers some of your questions.