Is it ok to tell a manager at work that she looks 98 y/o?

I really hope you're joking - although it's not funny at all.
If you're serious, you were incredibly rude, inappropriate and very unkind.
I suggest that you look for a new job and try to behave like a rational person in future.

You job jump frequently, don't you?  Great story, though.

I'd fire you if I was her.

Yes, of course it is very rude to tell that to a manager

By your own admittance, you wanted your Manager to feel self-conscious so ultimately, does this fit into your definition of this behavior being okay?  Only you can know your intention behind this decision-making.

It's not often people come on here and brag about being mean.

Er, No,  If she can take a joke you could say, 'Your'e no spring chicken!'.

NEVER!  Unless the person has bruises or scratches all over them - you do NOT say anything.  If anything, compliment her on doing a great job!

It's not a good idea to tell your manager that she looks as if she's 98 years old - not if you want to keep your job.

i wouldnt do that, thats rude

Bro that’s rude as hell, she probably hates you now