Sounds like toast to me.
The proper criteria for load testing a battery is to place it under a load three times its amp-hour rating for 15 secs and it should stay above 9.6v all the while.
Your car should probably have a 66ah battery which means a load of 198 amps. Your starter probably draws 150 amps so if you check while cranking for 15 seconds it probablt should not drop below 10v. The larger the load the quicker the drop and vise versa.
If its marginal I would replace it at that age.
Bosch Master, ASE Master, L1
33 years MB technician