I think it's a factor, but not an important factor. Elise Dopson is a prime example (you can look her up on Twitter). I think she's 21, maybe 22 now, but she's been blogging/writing/running a business since she was a teenager. Needless to say, she's young - but she makes six figures a year.
I think experience shows before age. If you can prove your value and experience, clients won't care if you're 15 or 45.
Do you have a portfolio? If not, Medium is a great place to keep writing. I've seen plenty of writers use their Medium collections as their portfolio.
If there are other publications you want to write for, see if they have guest posting opportunities. (You can find guest posting opportunities by Googling "psychology + guest post" or "psychology + write for us")
I rarely guest post anymore, but it was a big focus for me early on. I tried to get 10-15 guest posts to prove (a) I know what I'm talking about and (b) other people believe I know what I'm talking about (i.e. credibility). Then I'd share this content with potential clients.
Are you freelancing now? Or just considering it?