I am young and people treat my faith like a phase I will grow out of. How do I take it seriously when nobody around me does?

“Remember now thy Creator in the days of thy youth, while the evil days come not, nor the years draw nigh, when thou shalt say, I have no pleasure in them;”

Ecclesiastes is written by an old man who tried everything available to him, wealth, pleasure, work and wisdom, and found most of it thin. At the close he turns and speaks directly to the young, urging them not to wait. It is not a warning about wasting youth on fun. It is an older person's regret that the best years for building something with God are the ones easily assumed to be preliminary.

“Let no man despise thy youth; but be thou an example of the believers, in word, in conversation, in charity, in spirit, in faith, in purity.”

Timothy was leading a congregation in Ephesus that included people considerably older than he was, and he appears to have been anxious about it. Paul's answer is not to demand respect but to make the question irrelevant through how Timothy lives: speech, conduct, love, faith, purity. Credibility in this instruction is built rather than granted, and a young person can start building it today.