Well, I'm supposed to be a typical ENTP in his mid thirties, and yes my tertiary is Fe, and is getting stronger with each new year. And the trend is the same with every ENTPs I know.
Apollanaut description is interesting, while I'd say Nanook and Babylon Candle are simply speaking of things they don't understand, that are obviously too alien for them. Plus, the Socionic model never made any sense to me (from my experience, it was proven plain wrong too many times to really bother discussing it).
And yes, ENTPs are social, emotional animals who suffer from wild, unpredictable streams of irrationality (Ne-dom). And we like it that way, because it keeps us creative and enthusiastic.
Anyway, to keep the debate short:
1) When does this develop for you guys?
Only after I was 17 years old. Not really before.
2) Is it painful/difficult as it develops? (ie trusting others and so on..)
Initially, I developped my Fe thanks to painful events, but no, aside of them, the more this function develops, the more I feel balanced and at peace with myself.
3) Do you use it on purpose to influence others or does it emerge on its own at times?
Manipulating others is easy, and sometimes, well, quite interesting. I have to confess I do that quite often, especially at work (and feel no guilt about it). ENTP's iconic animal is the fox, you know. But Robin Hood is their patron saint.
That could also explain why I tend to be extremely generous even with perfect strangers. I feel the need to genuinely help, and there's no way I could explain why.
Since I have almost no Fi, it's not for moral reasons, I don't care about the means ("Morality makes light of morality" -> Pascal)... it just... happens!