Although CELTA and Trinity originated in the UK you can take the course in many countries at franchised schools.

1. Is CELTA only for adults?

Do you mean for teaching adults? If so, then I think it could be described as being so.

2. Does TEFL International really deserve their negative reputation? The comments left on their website from former students appear completely partial and honest, and yet didn't seem to be too negative.

Take a look on the forums and you'll get a general impression of their status within the industry. Also take a look at http://teflcoursereview.com which has reviews of courses by former students.

3. Would the best path by Trinity, or CELTA, or TEFL International?

It depends what for. If you're going for a first job then in my opinion CELTA or Trinity are too advanced. I strongly feel that a before taking either of these courses a teacher should have a couple of years experience teaching to make the most of them. They are intensive courses and without that experience it's going to be harder still.

For an entry level job you can take almost any decent TEFL certificate at very reasonable cost. Then work for a couple of years and then decide if you want to go for the CELTA. This is a common path taken by many teachers.