Yes, you can claim input tax credit on the old stock but the documents should not be old more that 12 months. Other conditions are as under.
Section 140(3) of the CGST Act 2017 -- A registered person, who was not liable to be registered under the existing law, or who was engaged in the manufacture of exempted goods or provision of exempted services, or who was providing works contract service and was availing of the benefit of notification No. 26/2012-Service Tax, dated the 20th June, 2012 or a first stage dealer or a second stage dealer or a registered importer or a depot of a manufacturer, shall be entitled to take, in his
electronic credit ledger, credit of eligible duties in respect of inputs held in stock and inputs contained in semi-finished or finished goods held in stock on the appointed day subject to the following conditions, namely:––
(i) such inputs or goods are used or intended to be used for making taxable supplies under this Act;
(ii) the said registered person is eligible for input tax credit on such inputs under this Act;
(iii) the said registered person is in possession of invoice or other prescribed documents evidencing payment of duty under the existing law in respect of such inputs;
(iv) such invoices or other prescribed documents were issued not earlier than twelve months immediately preceding the appointed day; and
(v) the supplier of services is not eligible for any abatement under this Act:
Provided that where a registered person, other than a manufacturer or a supplier of services, is not in possession of an invoice or any other documents evidencing payment of duty in respect of inputs, then, such registered person shall, subject to such conditions, limitations and safeguards as may be prescribed, including that the said taxable person shall pass on the benefit of such credit by way of reduced prices to the recipient, be allowed to take credit at such rate and in such manner as may be prescribed.