While the general disinterest of today’s young people in anything educational certainly hasn’t helped market prices, I would hesitate to recommend starting stamp/coin collections to anyone who is focused on profiting from them financially. I held onto many of my coins/stamps (that I myself had gotten for good prices at flea markets and such) because the offered prices were so low as the markets slid (and never recovered). Someday they may be worth something to my grandchildren, but they’ll probably be selling them to collectors from Red China or India.
If you do, more power to you, but it is just icing on the cake.