Closing oldest line of credit. Bad for credit score?

I've recently opened a current account with First Direct.

In order to qualify for their £100 switching incentive you have to use their full switching service which closes the old account.

I've had my other account with overdraft (HSBC) for 16 years. I read somewhere that you should not close your oldest line of credit as it can adversely affect your eligibility for credit.

I plan to get a mortgage in about three years. Is it likely to have an impact or should I just go for it and collect my £100?

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