A recent study has shown that it isn't about how long you have been fat for, rather your current weight at this specific time when trying to lose weight. Attempting to lose weight can start at any time and when you do health risks reduce significantly. Being obese at the age of 25 years old is shown to continue to be obese later on in life. Losing weight however means you will be healthier, happier and feel much better about yourself.
It is a daunting challenge when you want to lose weight but regular exercise and eating the right foods will certainly help you get there :)
Reference - Daily Mail - http://www.dailymail.co.uk/health/article-2620646/Its-never-late-lose-weight-improve-health.html