Definitely not too late! I just started speech therapy for the first time at age 30 after having a covert stammer all my life and at the first session my therapist said she was confident that I'd benefit from treatment. I'm 5 weeks in and I've already learnt so much about myself and how I feel about my stammer which I hadn't even realised, let alone acknowledged even to myself. I'm starting to develop techniques to improve my fluency, rather than avoiding difficult words and situations. It's definitely not true that how you speak at 25 is how you'll speak forever - that goes for stammerers and non-stammerers alike! Good luck!