In our experience here at The Rock Academy we feel that the best age to start guitar is around 9 years old. Finger strength plays a big part in this. The strength to play even a three quarter size nylon string guitar is quite a lot. Physically the guitar is the hardest of most contemporary instruments. Piano requires a lite touch, drums use large moto skills which kids learn from the age they crawl and singing is an extension of speaking.
This doesn't mean we can't make a start. If you are adamant you would like to learn guitar at the age of 5 then go for it! We have taught many five-year-olds to play and by the time they are 10, they are incredible. The first 3 or 4 years are very slow though and don't expect any practice outside of the lesson.
A great compromise is a ukulele! Super easy to play and we get results fast. Either way getting started early is the best. I started my kids at six and now they are both doing really well. My daughter plays in a band and my son is really motoring along now. All with minimal practice along the way.
The important thing is to have fun in your music lesson and build a habit of good feelings around music and musical instruments.