I don't think you could really guess that based on such a limited amount of text.

For what it's worth the most advanced kanji in this text, 値, is taught in sixth grade, the second most advanced is 価 which is taught in fifth grade... However this person could also be younger than a fifth or sixth grader and just learned those characters early due to repeat exposure, or they might be using a few characters that they don't actually know, just because they still know that that's the word they were trying to type; or this person could also be older than a sixth grader, but because they didn't use any words written with more advanced characters in the text, we can't know.

Aside from those two outliers, the remaining kanji in the text are divided equally between first-grade, second-grade, and third-grade. There's also a few words in the text that are spelled in hiragana that could've been spelled in kanji, but those are all second- or third-grade kanji as well, and those words are colloquially more commonly spelled in hiragana anyways, regardless of age.

But yeah, all in all trying to guess someone's age based on kanji usage is a lot like trying to guess someone's age based on vocabulary. With a large enough sample size coupled with really good deductive reasoning and statistical analysis, you could probably do it... But you can also really easily write multiple sentences in a row as a 70 year old, using only words you knew when you were 3.