Simply, I’d say don’t try to. You can hurt your voice pushing it too often into extremes of your register, especially when you’re young and your voice is still developing. I’m not actually a voice actor (I follow this sub for info and because I’d like to try acting at some point), but a classical singer… and one of the most important lessons we learn as younger singers is not to make our voices do things they find very difficult. Grown female voices often play younger male roles, but there’s a reason you don’t hear of the converse happening. I’d also wager that your ‘older man’ voice is unlikely to sound as convincing as the voices of any older people – make or female – simply because of the biology.