I am not sure if you require an aged account to create a wiki page but back in the days I created a wiki page on the day I created an account on the site. Creating a pages isn't rocket science.
Just make sure the content doesn't sound too 'salesy'. They will flag it as an advertisement right away.
Also add references to any facts that you write about. And make sure they aren't from your clients website or the same site. (Just a tip for this, I plan my pages months in advance and do blog posts or press releases that I can use later to improve my clients wiki pages. Even use some of our PBN;s for this. Wiki links!!

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Learn how to do the Company Infobox - that's the company data what appears on in a box format on the top right of the page.
You might also want to read up on uploading images. I was so confused when uploaded an image for the first time and about copyright status.
If your clients business is multilingual, it isn't difficult to create multi lingual pages.
And do a search for any mentions of your client on Wikipedia and get a link back to your page.
If your client does any work related to any other business or has partnerships worth mentioning, and those guys have wiki pages, you can always find a natural way to add a link from those pages to yours.
For example one of my clients sponsored a football club. I edit the wiki page of the football club, created a sponsorship section and got a link back to my page. Internal linking within wikipedia matters.
Don't get worried if you get flagged. Been flagged a few times. I created a politician profile a few years ago and a user flagged it. I messaged him and asked how I could improve it. I got some nice recommendations and improved the content in a couple of weeks.
Like i said before, it isn't rocket science. You can learn to do it yourself but
do your research. Wikipedia is a solid investment and works wonders if you spend enough time on it.