My stepmother recently got a prompt to renew her Norton anti-virus software for her phone—-$140!!! (she is in her upper 80’s). It does appear to be a real product but I do not know anyone who has it installed. She recently had a snafu with her email but that’s resolved and I don’t think it was related. I told her I would post here to be sure 
I presume that is an iphone? then I do not suggest to use an AV on iphone , but for Andriod. I think social engineering issue (like phishing, scam, et) is more to be aware of then viruses
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That Norton phishing scam is so common, and seems to strike seniors frequently, that the real Norton has some advice for victims of the scam
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Assuming the phone is running an up-to-date version of iOS, you don’t need anti-virus. The same is true for up-to-date Android, MacOS and Windows, all of which have built-in protection.
As @fuzzygel says, social engineering is far, far more likely to be an issue.