Hello to Fronkpies and other contributing members to this thread,
I can understand your concern about your ability to learn a new instrument such as the piano; your feelings are very real to you. While it is true that children under, say, the age of 12 can acquire piano playing skills faster, YOU have a quality about yourself that overrides what they have. Do you know what that is??? It is DESIRE, MOTIVATION!
Seeing that I currently teach 63 piano students per week (I am a retired metallurgical & materials engineer from the steel industry), I can give you some insight into the road ahead of you.
First some words of encouragement: Nearly everyone on the planet believes he or she is too old to start learning piano. Even some parents of 6 year olds "regret" they haven't introduced their children to piano lessons at age 3! Seventeen is a fine age to begin learning the piano. After all, you have perhaps another 75 to 85 years ahead of you to perfect your craft!
I tell my adult beginning students the following: The human mind is an amazing thing, because parts of the brain can be taught new things to do at any age in one's life. Take, for example, stroke victims who (God forbid) may have lost the ability to speak, or walk, or write with pen on paper. The affected portion of the brain has ceased functioning in its normal way.
Physical therapy takes the form of three activities: 1) showing the patient how to correctly perform a given task; 2) repetition, repetition, and repetition; and 3) positive reinforcement for each gain, no matter how small or large.
Something happens when a stroke victim is administered the correct therapy -- a DIFFERENT part of the brain, usually one that was NEVER intended for a given new function, happens to acquire the function lost previously by the stroke in a different part of the brain! This can occur spontaneously, especially with help from the three steps described above.
Do you know what, Fronkpies? The IDENTICAL method applies to piano lessons, when administered by a competent teacher!!!! So remember this -- if a stroke victim can re-acquire the ability to speak, or walk, etc., by means of the brain re-configuring itself, then playing the piano is literally child's play!
Now, down to the basics of learning to play the piano: Your ability to play the guitar has already demonstrated that you are aware of musical notes, rhythm, and hopefully the ability to read music. I would go one step further -- it is actually EASIER to play a note on the piano than it is to hold down a guitar string at the correct fret, and simultaneously excite the correct string with the other hands! In other words: One note on piano = one use of one finger VERSUS one note on guitar = combined effort of pressing the string with one hand, with sufficient downward force, and simultaneously coordinate the other hand to pick the correct string!!
What you gain in ease of making beautiful tones on a piano is offset by the usual requirement to play both the melody and accompaniment on the piano at the same time. Restated: Guitar = relatively few notes played at one time but with fair degree of difficulty VERSUS Piano = easy to play individual notes, but you usually have to play many more notes at the same time!
On practicing the piano -- I highly recommend that you find an instructor who directs you to perform far more hands separate practice than most teachers of beginning students. The more practice you can get, hands separately (AS A BEGINNER), I believe the faster you will progress. I would also leave you with these bits of information regarding practice:
Once you find a passage that is difficult to play, you will most likely determine that only two or three notes are the problem, rather than the whole passage. By all means, do NOT waste time repeating the entire passage, when you can concentrate your corrective action on the few notes that need correcting!
It is also a good idea, especially for a beginning student, to be able to play at a hands-separate tempo that is FASTER than performance tempo. Then when you put the two hands together, you will have already worked out the fingering problems, and your brain will have become acclimated to playing the piece!
Enough of my rambling.
If you find this reply helpful, please feel free to ask questions on this subject.