What is the ruling on using "Yahoo" service when the required legal age for its use is unknown, given that there is a condition prohibiting its use by those under the legal age?

First, read the website's terms and conditions to determine if they apply to you. If you strongly believe you have reached the legal age, then there is no harm in downloading it. However, if you know that you have not reached it, then you are not permitted to download the program, for the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions."