How to ask for information (text entry) with option not to answer

Hi I am asking participants for their age. I want to collect numerical data but also give participants the explicit option "Prefer not to say". I have done this using Multiple choice text entry but it looks messy as the text entry box is below the tick option and participants can in theory tick the box without entering any data. What is the best option to ensure a numerical response and have an explicit option to select if participants do not want to answer that looks tidy. Thanks, N