```python
from typing import Optional
from pydantic import BaseModel
from datetime import datetime

class CommentBase(BaseModel):
    content: str

class CommentCreate(CommentBase):
    pass

class CommentUpdate(CommentBase):
    pass

class Comment(CommentBase):
    id: int
    post_id: int
    user_id: int
    created_at: datetime
    updated_at: Optional[datetime] = None

    class Config:
        orm_mode = True
```

Explanation:

1. We import the necessary modules: `typing` for type hints, `pydantic` for defining data models, and `datetime` for working with date and time objects.


3. `CommentCreate` inherits from `CommentBase` and is used for creating new comments. It doesn't add any additional fields.

4. `CommentUpdate` also inherits from `CommentBase` and is used for updating existing comments. It doesn't add any additional fields.

5. `Comment` inherits from `CommentBase` and represents a complete comment object. It includes fields like `id`, `post_id`, `user_id`, `created_at`, and `updated_at`.

6. The `Config` class inside `Comment` sets `orm_mode = True`, which allows Pydantic to work with SQLAlchemy models and handle datetime objects correctly.