Well, most likely the only person here with any experience in this matter, I would say that it really depends on the individuals involved.
WARNING: LONG STORY
I was 16 when I met my track coach in high school (she was 25 at the time). I had transferred from another school my sophomore year and was already some what prominent in the San Diego area for the high hurdles. When I met my new coach (who's only experience was in distance running and was a girl), I was frankly quite despondent. I was sure that my parents had killed my track career just as it was beginning.
Still, there wasn't anything I could do about it so I decided to try and make the best of the situation. I had already been training year round for a few years and was pleasantly surprised when I found out that my new coach held summer workouts.
Being very eager, I showed up for our first workout early. We were all to meet at the beach and when I got there, my coach was already there too... sun bathing... in a bikini. She was incredibly sexy.
As the rest of the team showed up, she changed into her workout stuff and we did a distance run (which I hated, and pretty much ended me going to any further workouts). After our run my coach and I were standing around talking when she invited me over to her place for lunch. Being painfully shy around girls (specially sexy ones), I declined and rushed straight home.
I continued my workouts on my own until the next track season. I would check in with my coach, but pretty much disregarded anything she had to say as far as my workouts were concerned. We tended to argue quite a bit, but in the end I usually got my way.
After my first race that following track season I was tied for second in the county in the high hurdles, and my coach realized that I really did know what I was doing when it came to sprints and hurdles. In time she let me start coaching the other sprinters/hurdlers on the team. This meant that we ended up spending a lot of time together.
I can recall the first time she called me at home. It was for an upcoming invitational and she needed marks for the other athletes on the team and knew that I would know just about everyone's marks (even for the field events). It was rather cool actively helping with the team.
As it turned out, one of the hurdlers on the team was rather cute... though I never asked her out or anything, I paid quite a bit of attention to her. And I guess my coach noticed, though at the time I hadn't noticed her treating me any differently than before.
At the end of track season she shocked me by saying that she really didn't want to spend time together anymore. I was hurt, but I was more confused... why should I care if someone who doesn't even know my event discusses it with me.
Shortly after that that a sprint/hurdle coach at SDSU spotted me and offer to coach me. This was great as he had just coached his sister to an American record in the 400 meter hurdles. All of a sudden I was getting world class coaching and working out with world class athletes... plus his sister was really cute!
At the beginning of school my senior year I went to my coach to, well, gloat about my summer and my new coach. I was expecting one of our fights, but strangely she just listened to me talk about my summer. Then she took me completely by surprise by asking me to teach her how to hurdle.
We spent every day together after that. At first just working out, but then we started going out to lunch or having dinner, or I would go with her to her college and workout on their track while she was in class.
One day in December while she was driving me home she asked if I knew why she had stopped talking to me the previous year. I told her I didn't and that I had been quite hurt by it. She told me that she had gotten jealous of me spending so much time with one of the athletes. By the time I got home I had told her I loved her. And a week later she told me she loved me too.
Oddly, my parents were very supportive of us, including double dating with us from time to time.
Towards the end of the season while she and I were working out at SDSU to get ready for CIF championships, she was sitting on a steeple chase barrier talking with another coach who was there with his star athlete. I saw them talking as I past them on one of my runs, but when I reached the finish and turned around, my coach was on the ground. She had fallen back and her knee hit her nose when she landed. Her nose was broken, but there wasn't much I could do... I didn't know how to drive.

So she drove us to the hospital.
There wasn't anything they could do at the time, so we went home and waited for my parents. My mother found a doctor for her (she needed rhinoplasty), and we set her up in our guestroom for the next few weeks.
During the championships she had to sit with my parents in the spectator's bleachers, and we would have athletes run up to see her for last minute advice before their events.
After track season my coach moved in with her father who was dying of cancer. I tried to see her as much as possible, but her schedule was rather full... though not so full that she didn't take time to teach me how to drive!
One night I wasn't able to get a hold of her and started getting quite worried. I was still learning how to drive, but I backed my mother's car out onto the street and started it up and set out to find her. I had been to her father's a couple times... with her driving and in the day light, but after a couple hours of searching I found the place.
Everything was okay and she was sleeping in one of my shirts (it was really cute!). Thankfully she gave me directions back home.
After her father died she moved back in with us for about a month while we hunted for an apartment. We moved in together, lived together for about a year and a half and then were married for 9 years.
So, do I regret any of that... no, not really. Other than the last year or so that we were together, those were the happiest years of my life.
Still, as I approached her age (between 25 and 30) I started to realize just how young someone in their teens really is. While it wasn't that big a deal for me back then, it must have been for her... and even though I wasn't an average teenager (and didn't have much in common with other teenagers), I was still a teenager none the less.
Oh well, make of it what you will. But for me, it is hard to judge others in a similar circumstance without knowing that everyone is different.