What is an acceptable percentage for receivables aged more than 90 days?

Depends on industry.  Also, are you B2C or B2B?

No two companies are alike.  It all depends on how aggressively you
pursue collection efforts on these past due accounts.

For example, if your company is 20 years old (and have a lot of
historical data to perform an analysis on), and you have 'never'
collected a single penny on any accounts past 90 days, the answer
would seem fairly obviously.  You would set up a reserve for bad debts
that will equal 100% of your past due accounts (over 90 days).  This
would be based on your historical collection rate, which in this
extreme example would be zero (i.e. zero percent probability of
collection).

In general terms, I would look at historical bad debt collection rates
and the costs associated (i.e. collection agency fees) to determine a
historical collection ratio.  Then apply that ratio to your accounts
over 90 days.  Reassess these ratios quarterly and make adjustments
accordingly.

Hope that helps. =)

There is not enough informmation in your question to give you a true
answer. Without more information your answer can only be given by
speculating.
Knowing if your receivables are b2b and b2c is only the beginning
factor. The beginning factor is what is your credit extension policy?
Do You have any? What is your average account balance. What is your
billing practice? Who are your customers? and then then the type. In
consumer lending, demographics are going to be an important factor.
What is the unemployment rate? Dont use the National figure if your
are in an area like El Centro, CA where unemployment is almost always
above 30%.

Suggest you contact some professional help. Try MasTon Capital at
http://www.MasTonCapital.com they have helped credit grantors recover
over $500 million dollars since 1973.