Simply put, the answer is no. Getting braces, or the braces you already have will not loosen your teeth to the point that they will fall out. Braces are used to correct major and minor problems people face with their teeth and the positioning of their teeth. They pull on your teeth and shift them into the correct positioning which might make them feel a little loose at times, but they are not.
During the first few days after your braces are fitted, your teeth will begin to move a tiny amount and often they will feel rather loose. This is completely normal and a normal pattern of your tooth movement. This is actually the method your teeth use to move through your braces. After getting braces you may feel as though your teeth are getting loose and you may even see an opening or space where there was not one before. This too is a normal effect of the braces shifting your teeth into the correct position. This may cause concern for you if it was not addressed or explained by your orthodontist prior to having the braces put on.
Research has actually shown that tooth movement may cause small changes in the length and shape of your roots. In most cases, however, these changes are so small that they are not even visible to your bare eye. Although in less than 2% of people, some root shortening occurs that is visible. This happens during the standard routine orthodontic treatment. Root shortening is also known as tooth resorption and it is currently unknown what the cause is of tooth resorption. It is widely believed that genetics is the reasoning behind this but that is currently unproven. Generally, this does not result in any long-term problem for your teeth though due to it being believed that your teeth can actually loosen up to half of each individual root length without having any problems.
If you have orthodontic problems, you may benefit from braces regardless of what your age is. Although the ideal time for the placement of braces on teeth is between 10 and 14 years of age, you are able to get braces at any age and may see great results from them. More and more adults and young adults are also getting and wearing braces to help them correct minor dental problems and to help improve their smile.
To conclude, your braces will not actually make your teeth noticeably loose. However as stated above in some rare cases of root shortening your orthodontist may monitor your progress and make recommendations to you accordingly depending on your progress. If you currently have braces or are getting them soon always be sure to communicate any thoughts, questions or concerns you might have to your orthodontist. This way you will be able to address any problems or questions you have rather than pondering them over in your head wondering but unsure.