Seeking knowledge when one is older
What I mean here is seeking knowledge in a methodical manner as explained by our most prominent shaykhs such as Shaykh ibn ‘Uthaymeen in his book al-‘Ilm. My aim in acquiring knowledge is first to ward off ignorance from myself, and to worship Allaah with knowledge, then I will try to dispel the ignorance of those who are around me.
You should note that the greatest blessing that Allaah can bestow upon His slave is to guide him out of misguidance and save him from going astray. When Allaah opens the heart of His slave to acquire and teach Islamic knowledge, this is a great honour that Allaah bestows upon him. This makes the person more grateful towards his Lord and makes him strive to please Him. That includes striving to acquire the knowledge that has been handed down from the leader of the Messengers (peace and blessings of Allaah be upon him).
With regard to what you mention about being twenty-five years old, what does it matter?
You should strive to memorize the Book of Allaah and whatever you can of the Sunnah of the Messenger of Allaah (peace and blessings of Allaah be upon him) and useful texts. You should read and study the books of the scholars, and keep company with righteous shaykhs whose ‘aqeedah and understanding are sound, and who follow the methodology encouraged by scholars such as Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) and other prominent scholars.
In this website and in the al-Mukhtaar al-Islami website, you will find a section devoted to knowledge and the pursuit thereof. You should read these sections, for there you will find, by Allaah's leave, many answers to the questions that may cross your mind about this topic.
Remember that the path to knowledge is long, but it is easy for the one for whom Allaah makes it easy. So strengthen your resolve and seek the help of your Lord, and do not worry about your age. Perhaps what is left of your life will be better for you than what has passed, by Allaah’s leave.
Remember that the greatest companions of the Messenger of Allaah (peace and blessings of Allaah be upon him) did not acquire knowledge until they started to get grey hairs, and they were older than you are now. It did not take long until they became masters of knowledge and worldly leaders. In them is an example for everyone. There is the example of al-‘Izz ibn ‘Abd al-Salaam and Ibn Hazm and others who started to seek knowledge at and older age. So seek the help of Allaah and do not feel helpless.
We ask Allaah to guide you and help you.
And Allaah knows best.