Welcome! <3

1) Males will automatically be classed as a Submale & need a Submale slot to stay with you at 2 years old. They can still be sex-changed using an Adult Sex Changer until 3 years old, as long as they are not patrolled & non-mutated (the last day to sex-change a mutie is 1yr11mo old).

You don't have to consider them 'submale' or use them as a submale, but they count as one for the game's territory count purposes unless sex-changed into a lioness before reaching 3 years old in-game. Adolescent males aging up will produce a warning in your den page/sometimes the Rollover message at 1yr11mo if you have no space for them same as adolescent females.
I personally keep a few more Submale slots than the submales I actually want to keep (usually 1 heritageless male Groupie i'm training as an heir &/or a Looks heir if desired) so I usually only have 1/3 or 2/3 max Submale spaces filled so that I have some flexibility with aging males in case I need a little more time to decide/gather funds...
I've forgotten to change a cute mutie who i'd have liked as a female before, so it's a good idea to keep an eye on them & keep the requirements in mind if you produce something really pretty that's worth more than the 1gb sex-change fee to you! <3
You could technically use a Safou fruit from Monkey Business to remove the mutation if you really really don't care/wanted to keep him as a female & just forgot, but it's rarely done. :D (You can't get the mutation back afterwards unless there's an applicator for it, and not at all if it's gender-specific, apps generally are not used for pride members due to their value.)

2) Personally? I usually sex-change the prettiest ones if I need to keep them for my breeding projects (1 GB can add up fast, so quickly you'll find yourself increasing your standards until keeping a male cub requires a few more traits than your keepable females <3).

3) No, it does not! You can King any male lion, those below 3 years old will age up to 3 years upon Kinging & those above 10 years, 1 month old will age down to 8 years old.
The last day to King a Submale is 14yr11mo (Essentially) as he has a chance of dying upon reaching 15 years old, so be sure to use your older submales before then if you want. :D

Hope this helps! <3

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